Method for forming oxide film on metal surface using ionic liquid, electrolytic capacitor and electrolyte thereof

ABSTRACT

The present invention provides means for forming an oxide film on a metal surface, means for repairing a defect of an oxide film, a high-performance electrolytic capacitor using the means, and an electrolyte of the capacitor. Namely, the prevent invention provides a method for easily forming an oxide film on the surface of a metal or an alloy thereof by anodization using a solution containing an ionic liquid. In an application of this method, an electrolytic capacitor having means for repairing a defect of an oxide film can be formed by a method using, as an electrolyte, an ionic liquid, a solution containing an ionic liquid and a salt, or a solution containing an ionic liquid and a conductive polymer or a TCNQ salt, and a valve metal or an alloy thereof as a metal.

TECHNICAL FIELD

The present invention relates to a method for forming an oxide film on ametal surface by anodization or a method for repairing a metal oxidefilm, an electrolytic capacitor using the principle of the formation orrepair of an oxide film by the method, and an electrolyte of thecapacitor.

BACKGROUND ART

An anodization method is a method for forming an oxide film on thesurface of a metal used as an anode in an acid solution or a neutralsolution. This method is frequently used for forming oxide films onvalve metals such as, aluminum and tantalum. For example, with aluminum,porous thick oxide films are formed in an acid solution of sulfuricacid, oxalic acid, phosphoric acid, or the like and barrier-type densethin films are formed in a neutral solution of a borate, a phosphate, anadipate, or the like. Porous aluminum oxide films are used for corrosionprevention, friction prevention, coloring decoration, and the like, andbarrier-type films are widely used as dielectrics of electrolyticcapacitors.

An electrolytic capacitor generally includes an anode composed of avalve metal such as aluminum or tantalum, a dielectric composed of anoxide film formed on the surface of the anode, and a cathode formed tohold an electrolyte between the cathode and the dielectric. In theelectrolytic capacitor, the driving electrolyte has two importantfunctions. One of the two is the function as the actual cathode. Namely,the electrolyte functions to extract a capacitance from the dielectricformed on the anode and is required to have high electric conductivity,i.e., high electron conductivity. The other is the function to protectand repair a very thin oxide film, i.e., the chemical function to form anew oxide in a defect of an aluminum or tantalum oxide film on the basisof the ion conductivity possessed by the electrolyte. Namely, theanodization is used for forming a dielectric oxide film in anelectrolytic capacitor and for repairing a defect of an oxide film.Therefore, the electrolyte of the electrolytic capacitor is required tohave an anodizing ability.

As the electrolyte of the electrolytic capacitor, an organic solventsuch as ethylene glycol or γ-butyrolactone containing an organic acid,an inorganic acid, or a salt thereof is generally used. Specificexamples of an organic acid, an inorganic acid, or a salt thereof addedto the solvent include phosphoric acid, formic acid, acetic acid,ammonium adipate, ammonium succinate, tertiary amines, and quaternaryammonium salts. Such a composite electrolytic system is used for formingan electrolyte having excellent ion conductivity (Patent Document 1).

Although the conductivity of such a liquid electrolyte is improved byadding the above-described additive, the conductivity is only about 10⁻³S/cm, which is unsatisfactory for realizing a low-impedance capacitor.Also, the liquid electrolyte causes a dry-up phenomenon due toevaporation of the solvent used, and both the anodizing property andconductivity are lost by the dry-up. Therefore, the electrolyte isunsatisfactory in the long-term life and heat resistance.

In order to improve these properties, use of a molten salt as anelectrolyte for a capacitor has been investigated. For example, aninvestigation has been conducted for forming an electrolyte for acapacitor by melting or melting and then solidifying an electrolyticsalt having a nitrogen-containing heterocyclic cation having aconjugated double bond or a nitrogen-containing heterocyclic ringcontaining a conjugated double bond without using a solvent (PatentDocument 1).

Also, an investigation has been conducted for forming a capacitorincluding an electrolyte for an electrolytic capacitor interposed singlyor together with a separator between an anode foil and a cathode, theelectrolyte being prepared by melting a mixture of a carboxylate and acarboxylic acid without using a solvent (Patent Document 2). However,these electrolytes are solid at room temperature and thus have the verylow anodizing ability and low conductivity. Therefore, the electrolyteshave been not yet put into practical applications.

On the other hand, solid capacitors not containing a solvent have beenrecently developed. Specifically, these capacitors each include, as anelectrolyte, a conductive polymer, such as polypyrrole, polyaniline, ora polythiophene derivative. Since these conductive polymers haveextremely higher electric conductivity (electron conductivity) than thatof the above-described conventional electrolytic solutions eachcontaining an electrolyte and a solvent, the internal impedance of acapacitor using such a conductive polymer as an electrolyte can bedecreased. In particular, when these conductive polymer capacitors areused for high-frequency circuits, excellent properties are exhibited.Therefore, such conductive polymer capacitors are establishing animportant position in the electrolytic capacitor market.

However, conductive polymers basically do not have ion conductivity, andthus conductive polymer capacitors are far inferior to conventionalcapacitors each including an electrolytic solution in the anodizingfunction to repair oxide films of electrolytic capacitors. It isgenerally said that in a conductive polymer capacitor, a conductivepolymer present on the dielectric surface of a damaged portion isinsulated by the de-doping reaction of the conductive polymer due to theJoule heat generated in damage to a dielectric film, thereby preventingthe breakage of the dielectric film. Such a mechanism is fundamentallydifferent in principle from a mechanism occurring in the function torepair an oxide film of a conventional capacitor using an electrolyticsolution (Non-patent Document 2).

Consequently, the conductive polymer capacitors are disadvantageous inthat capacitors with a high withstand voltage cannot be formed.Specifically, under present conditions, when aluminum is used for ananode, a conductive polymer capacitor having a withstand voltage up toonly about 16 V can be produced, for example, with a formation voltageof 70 V, and when tantalum is used, a conductive polymer capacitorhaving a withstand voltage up to only about 12 V can be produced, forexample, with a formation voltage of 34 V. The term “formation voltageof 70 V” means that in forming a dielectric oxide film on a valve metalsurface, the DC voltage applied to the valve metal, i.e., the formationvoltage, is 70 V. Of course, the withstand voltage can be basicallyincreased by increasing the formation voltage. In this case, however,the capacitance of a capacitor decreases as the formation voltageincreases, and the withstand voltage does not increase in proportion toincreases in the formation voltage. Therefore, this is not said to be apreferred method.

As an attempt to improve the withstand voltage characteristic of aconductive polymer capacitor, an electrolytic capacitor is disclosed, inwhich an electrolyte including a conductive polymer and an organic acidonium salt is used (Patent Document 3). However, it is assumed that theorganic acid onium salt is basically a solid salt. Therefore, in orderto improve the withstand voltage, the ratio between the conductivepolymer (A) and the organic acid onium salt (B) is thought to bepreferably in a range of (A):(B)=1:0.1 to 5 and more preferably in arange of (A):(B)=1:0.2 to 2. However, at a ratio in this range, thewithstand voltage is certainly improved, but the conductivitycharacteristic degrades, undesirably resulting in the deterioration inthe impedance characteristic of the capacitor. Apart from theabove-described technique relating to electrolytic capacitors, a moltensalt in a liquid state at room temperature have been developed andattracted attention. The molten salt is referred to as an “ionicliquid”, includes a combination of a quaternary salt cation, such asimidazolium or pyridinium, and an appropriate anion (Br⁻, AlCl⁻, BF₄ ⁻,or PF₆ ⁻), and frequently contains a halogen. The ionic liquid has theproperties such as nonvolatility, noninflammability, chemical stability,high ion conductivity, and the like, and attracts attention as areusable green solvent used for various syntheses and chemical reactionssuch as catalytic reaction. However, there has been no example ofinvestigation of the ionic liquid from the viewpoint of the anodizingproperty, i.e., from the viewpoint of the formation of an oxide film ona valve metal surface or the repair of an oxide film.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 5-13278

Patent Document 2: Japanese Unexamined Patent Application PublicationNo. 5-101983

Patent Document 3: Japanese Unexamined Patent Application PublicationNo. 2003-22938

Non-patent Document 1: Denkai Chikudenki Hyoron (Electrolytic CondenserReview), Vol. 53, No. 1, p. 101 (2002)

Non-patent Document 2: Denkai Chikudenki Hyoron (Electrolytic CondenserReview), Vol. 53, No. 1, p. 95 (2002)

DISCLOSURE OF INVENTION

Considering the above-described situation, the inventor of the presentinvention has found that an ionic liquid causing no dry up due toevaporation has an excellent oxidation property, leading to theachievement of the present invention. Namely, the present inventionrelates to a method for easily forming an oxide film on a metal surfaceby anodization in the presence of an ionic liquid or a method forrepairing a previously formed metal oxide film, and an electrolyticcapacitor in which the dielectric forming and repairing ability issignificantly improved by utilizing the oxide film forming ability ofthe method. Furthermore, the present invention can realize the excellentelectron conductivity and oxide film repairing ability by combining theionic liquid with an electrolyte for a solid electrolytic capacitor,such as a conductive polymer electrolyte or a TCNQ salt electrolyte, andcan thus form an electrolytic capacitor with a low impedance and a highwithstand voltage.

The present invention includes the following forms:

1. A method for forming an oxide film on a metal surface, in whichanodization is performed in the presence of an ionic liquid.

2. The method for forming an oxide film on a metal surface describedabove in 1, in which a defect of an oxide film previously formed on ametal surface is repaired by anodization in the presence of an ionicliquid.

3. The method for forming an oxide film on a metal surface byanodization described above in 1 and 2, in which the metal is at leastone selected from aluminum and/or alloys thereof, tantalum and/or alloysthereof, and niobium and/or alloys thereof.

4. The method for forming an oxide film on a metal surface describedabove in 1 to 3, in which an anion component of the ionic liquid is anatomic group containing fluorine.

5. The method for forming an oxide film on a metal surface describedabove in 1 to 3, in which an anion component of the ionic liquid is anatomic group containing a sulfonic acid anion (—SO₃ ⁻).

6. The method for forming an oxide film on a metal surface byanodization described above in 1 to 3, in which an anion component ofthe ionic liquid is an atomic group containing a carboxylate anion(—COO⁻).

7. The method for forming an oxide film on a metal surface describedabove in 1 to 6, in which a cation component of the ionic liquid is atleast one selected from imidazolium derivatives, ammonium derivatives,and pyridinium derivatives.

8. The method for forming an oxide film on a metal surface byanodization described above in 1 to 7, in which a solution containingthe ionic liquid and at least one selected from ammonium salts, aminesalts, quaternary ammonium salts, tertiary amines, and organic acids isused.

9. An electrolytic capacitor including means for the method describedabove in 1 to 8, for repairing an oxide film.

10. An electrolytic capacitor including a solution containing at leastone ionic liquid, the solution being used as an electrolyte serving asmeans for repairing an oxide film.

11. The electrolytic capacitor described above in 10, in which thesolution further contains a conductive polymer.

12. The electrolytic capacitor described above in 11, in which theconductive polymer is at least one selected from polypyrrole,polyaniline, polythiophene, and derivatives thereof.

13. The electrolytic capacitor described above in 11 and 12, in whichthe weight ratio (ionic liquid/conductive polymer) of the ionic liquidto the conductive polymer is in a range of 1/10,000 to less than 1/10.

14. The electrolytic capacitor described above in 10 to 13, in which thesolution further contains a TCNQ salt.

15. The electrolytic capacitor described above in 14, in which the TCNQsalt is a salt containing a donor composed of a nitrogen-containingheterocyclic compound substituted by an alkyl at the N position and anacceptor composed of TCNQ.

16. The electrolytic capacitor described above in 11 to 15, in which ananion component of the ionic liquid is an atomic group containing atleast fluorine.

17. The electrolytic capacitor described above in 10 to 15, in which ananion component of the ionic liquid is an atomic group containing atleast a sulfonic acid anion (—SO₃ ⁻),

18. The electrolytic capacitor described above in 10 to 15, in which ananion component of the ionic liquid is an atomic group containing atleast a carboxylate anion (—COO⁻)

19. The electrolytic capacitor described above in 14 to 18, in which theweight ratio (ionic liquid/TCNQ salt) of the ionic liquid to the TCNQsalt is in a range of 1/10,000 to less than 1/2.

20. The electrolytic capacitor described above in 10 to 19, in which acation component of the ionic liquid is an imidazolium derivative, anammonium derivative, or a pyridinium derivative.

21. An electrolyte including a solution containing the ionic liquiddescribed above in 1 to 8, in which the electrolyte is used for formingan oxide film on a metal surface by anodization.

22. An electrolyte including a solution containing the ionic liquiddescribed above in 9 to 22, in which the electrolyte is used for anelectrolytic capacitor.

The means described above in 1 was achieved on the basis of the findingby the inventor that an ionic liquid has an excellent metal oxidizingability. The means described above in 2 permits the solution containingthe ionic liquid to repair a defect of a metal oxide film previouslyformed by another method. The means described above in 3 to 7 canexhibit a particularly excellent ability of forming and repairing anoxide film on a valve metal surface. The means described above in 8 iscapable of controlling the ability of forming an oxide film on a valvemetal surface and the ability of repairing an oxide film. The meansdescribed above in 9 relates to an electrolytic capacitor using themethod for forming an oxide film on a metal surface by any of theabove-described anodization means or the method for repairing a defectof an oxide film. The means described above in 10 relates to anelectrolytic capacitor using the above-described method for forming anoxide film on a valve metal surface and the method for repairing adefect of an oxide film. The means described above in 11 to 13 relate toa high-performance capacitor having the excellent electron conductivityof the conductive polymer and an oxide film repairing property based onthe excellent ion conductivity of the ionic liquid, and having excellentimpedance characteristics. The means described above in 14 and 15 relateto a high-performance capacitor having the excellent electronconductivity of the TCNQ salt and an oxide film repairing property basedon the excellent ion conductivity of the ionic liquid, and havingexcellent impedance characteristics. The means described above in 15 to20 can exhibit a particularly excellent ability of forming and repairingan oxide film on a valve metal surface. The means described above in 21and 22 relate to an electrolyte which imparts the ability of forming andrepairing an oxide film on a valve metal surface to an electrolyticcapacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows typically observed changes in current when an electrolytehas an ability of repairing a metal oxide film.

FIG. 2 shows changes in current in re-formation (oxide film repairingexperiment) using ILS-1 under the following conditions: an initialformation voltage of 200 V, a rate of voltage rise of 1 V/sec, and ameasurement temperature of room temperature.

FIG. 3 shows changes in current in re-formation (oxide film repairingexperiment) using an aqueous adipic acid solution (1 g/l) under thefollowing conditions: an initial formation voltage of 200 V, a rate ofvoltage rise of 1 V/sec, and a measurement temperature of roomtemperature.

FIG. 4 is a conceptual drawing of an electrolytic polymerizationapparatus for a conductive polymer.

REFERENCE NUMERALS

-   -   A: start point of decrease in current by re-formation reaction    -   B: end point of re-formation    -   C: point indicating withstand voltage of electrolyte    -   1: polymerization initiating electrode    -   2: aluminum foil    -   3: dielectric layer    -   4: manganese oxide conductive layer    -   5: conductive polymer layer    -   6: electrolytic solution    -   7: cathode

BEST MODE FOR CARRYING OUT THE INVENTION

As a result of various investigations for resolving the above-descriedproblems, the inventor of the present invention has found that a seriesof compound groups referred to as “ionic liquids” exhibits an excellentanodizing property, resulting in the achievement of the presentinvention.

An ionic liquid used in the present invention is referred to as a“room-temperature molten salt” and is liquid at room temperature inspite of being composed of an anion component and a cation component.Unlike ordinary organic solvents, an ionic liquid is not partiallyionized or dissociated but is composed of only ions. In other words, itis thought that the ionic liquid is 100% ionized. Although the term“ionic liquid” generally means a liquid at room temperature, the ionicliquid used in the present invention is not necessarily liquid at roomtemperature as long as it becomes liquid by aging or heat treatment of acapacitor and spreads over the whole of an electrolyte of the capacitoror becomes liquid by the Joule heat generated in a repair of an oxidefilm. As a cation used for the ionic liquid suitable for the object ofthe present invention, various cations having quaternary nitrogen can beused. Examples of the cation include ammonium and its derivatives,imidazolinium and its derivatives, pyridinium and its derivatives,pyrrolizinium and its derivatives, pyrrolinium and its derivatives,pyrazinium and its derivatives, pyrimidinium and its derivatives,triazonium and its derivatives, triazinium and its derivatives, triazinederivative cations, quinolinium and its derivatives, isoquinolinium andits derivatives, indolinium and its derivatives, quinoxalinium and itsderivatives, piperazinium and its derivatives, oxazolinium and itsderivatives, thiazolinium and its derivatives, morpholinium and itsderivatives, and piperazine and its derivatives. In particular,imidazolinium derivatives, ammonium derivatives, and pyridiniumderivatives can be preferably used for the object of the presentinvention. Here, the term “derivatives” means having substituents suchas hydrogen, aliphatic hydrocarbon groups, alicyclic hydrocarbons,aromatic hydrocarbons, carboxylic acids, ester groups, ether groups,acyl groups, and amino groups. Any position of the cation component issubstituted by such a substituent.

As the anion component preferably used for the object of the presentinvention, fluorine-containing anions can be used. Examples of theanions include BF₄ ⁻, PF₆ ⁻, and R_(A)SO₃ ⁻ (wherein R_(A) represents afluorinated substituent containing a fluorinated aliphatic hydrocarbongroup, a fluorinated alicyclic hydrocarbon group, a fluorinated aromatichydrocarbon group, an ether group, an ester group, an acyl group, or thelike). Specific examples include CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, (CF₃SO₂)₃C⁻,CHF₂CF₂CF₂CF₂CH₂OSO₃ ⁻, CHF₂CF₂CF₂CF₂CH₂SO₃ ⁻, and the like. Thesesubstituents can be preferably used for the object of the presentinvention, and BF₄ ⁻ is also preferably used as the anion for thepurpose. Of course, the fluorine anion suitable for the presentinvention is not limited these examples.

As the anion component preferably used in the present invention, anatomic group including a sulfonic acid anion (—SO₃ ⁻) can be used. Theatomic group is represented by R_(B)SO₃ ⁻ (R_(B) represents asubstituent containing an aliphatic hydrocarbon group, an alicyclichydrocarbon group, an aromatic hydrocarbon group, an ether group, anester group, an acyl group, or the like) and may contain fluorine.Specific examples of the atomic group include pCH₃C₆H₄SO₃ ⁻, C₆H₅SO₃ ⁻,CH₃CH₂OCH₂CH₂OSO₃ ⁻, C₆H₅OCH₂CH₂OSO₃ ⁻, and the like. In particular,anions containing both fluorine and a sulfonic acid anion are preferablyused for the object of the present invention. Specific examples of suchanions include CHF₂CF₂CF₂CF₂CH₂OSO₃ ⁻, CHF₂CF₂CF₂CF₂CH₂SO₃ ⁻, and thelike. Of course, the sulfonic acid-containing anion suitable for thepresent invention is not limited to these examples.

As the anion component preferably used in the present invention, anatomic group containing a carboxylate anion (—COO⁻) can also be used.Specific examples of such an atomic group include R_(C)COO⁻,⁻OOCR_(C)COOH, ⁻OOCR_(C)CCOO⁻, and NH₂CHR_(C)COO— (wherein R_(C)represents a substituent containing an aliphatic hydrocarbon group, analicyclic hydrocarbon group, an aromatic hydrocarbon group, an ethergroup, an ester group, an acyl group, or the like). Of course, theatomic group may contain fluorine. Specifically, it is effective tosynthesize the ionic liquid containing a carboxylate anion (—COO⁻) usingformic acid, acetic acid, maleic acid, adipic acid, oxalic acid,phthalic acid, succinic acid, or an amino acid. Of course, thecarboxylate (—COO⁻) suitable for the present invention is not limited tothese examples.

The anion preferably used in the present invention can be furtherexemplified by NO₃— and R_(d)NO₃— (wherein R_(d) represents asubstituent containing an aliphatic hydrocarbon group, an alicyclichydrocarbon group, an aromatic hydrocarbon group, an ether group, anester group, an acyl group, or the like, and may contain fluorine).

Among zwitterion-type ionic liquids each containing a cation and ananion bonded together by a covalent bond, an ionic liquid containing asulfonic acid anion or an ionic liquid containing fluorine can bepreferably used for the object of the present invention. The ionicliquid of the present invention includes a combination of the anion andthe cation and can be synthesized by a known method. Specifically, amethod, such as an anion exchange method, an acid ester method, or aneutralization method, can be used.

Next, an anodization process using the ionic liquid of the presentinvention will be described. The anodization process is widely used asmeans for forming a metal oxide film on a metal surface by applying avoltage or a current in an electrolytic solution using, as an anode, ametal on which the metal oxide is to be formed. This process is the mostgeneral process as means for forming an oxide film on the surface of avalve metal, particularly, aluminum, tantalum, niobium, or the like.Although the method for forming an oxide film of the present inventionwill be descried with reference to an example using aluminum, the methodcan be applied to other valve metals, such as tantalum and niobium. Inaddition, the method can be applied to aluminum and/or its alloys,tantalum and/or its alloys, niobium and/or its alloys, and other metalsin the basically same manner. The application range of the presentinvention is not limited to aluminum, and the present invention can beapplied to valve metals, such as tantalum and niobium.

The anodizing ability of an electrolytic solution is measured asfollows: A cell including an aluminum anode and a stainless steel,copper or platinum cathode is immersed in the electrolytic solution, anda predetermined voltage is applied between the electrodes to measurechanges in the current flowing between the electrodes. The appliedvoltage may be increased at a constant rate to measure changes in thecurrent. Namely, when an insulating oxide film is formed on a metalsurface due to the oxide film forming ability of the electrolyte, nocurrent flows. However, the oxide film forming ability of theelectrolyte is limited, and the formed oxide film becomes impossible toresist the voltage with increases in the voltage, finally resulting inbreakage of the film. The anodizing ability of the electrolyte can beestimated by measuring such a change in the current.

On the other hand, in order to evaluate the metal oxide film repairingability of an electrolytic solution, it is advantageous to use ananodized film prepared in an existing electrolytic solution with apredetermined voltage, the film being partially defected by apredetermined method such as boiling the oxide film in boiling water.The specimen prepared by this method may be immersed in the electrolyticsolution to be evaluated, and the voltage may be increased at apredetermined rate to measure changes in the current. This is referredto as a “re-formation evaluation method”. In this method, the formationvoltage of the previously formed anodized film is selected (i.e., thethickness of the anodized film is changed) so that the same experimentas the above-described oxide film formation experiment can be carriedout. In other words, for example, when the oxide film is formed at 100V, the anodizing ability of the electrolytic solution can be measured byobserving the breakdown voltage (V) in the same experiment as theabove-described oxide film formation experiment.

Therefore, the latter experiment of repairing a metal oxide film can beconducted as the former experiment of evaluating the anodizing ability.This applies to the device evaluation of an electrolytic solution for acapacitor of the present invention. Therefore, most evaluations are madeby the latter method. When the electrolytic solution has the metal oxidefilm repairing ability, typically observed changes in the current are asshown in FIG. 1.

First, the current flows through a defected portion of an oxide film(region 1). When the electrolytic solution has the anodizing property, anew oxide film is formed in the defected portion by the film repairingability of the electrolytic solution, thereby decreasing the currentbeyond the maximum (A) (region 2). The minimum point (B) of the currentrefers to the end point of repair, and a linear current increase regionproportional to increases in the voltage appears after the minimum point(B) (region 3). When the voltage is further increased, however, thecurrent deviates from a linear relation and starts to flow at a certainvoltage (C) (region 4). This indicates the actual withstand voltage ofthe electrolytic solution and corresponds to the above breakdownvoltage. When the electrolytic solution has no anodizing ability, ofcourse, only the region 1 appears, and the current continuously flows tofinally cause breakage of the oxide film.

There are two types of aluminum anodized films including a barrier-typedense film and a porous film. A barrier-type dense film is produced in aneutral solution of a borate, a phosphate, or the like, and a porousfilm is produced in an acid solution such as an aqueous solution ofphosphoric acid, sulfuric acid, or oxalic acid. The porous film isproduced due to local dissolution of the film during anodization. Whenthe formation of such a porous film is started by local dissolution,protons in the solution enter the film due to a thermal function againstan electric field, thereby starting a flow of a large quantity of ioncurrent. In FIG. 1, at a voltage higher than point (C), the currentincreases due to a rapid increase in the ion current, and the increasepoint of the current is preferably as high as possible. Therefore, theanodizing ability of the electrolytic solution can be evaluated bymeasuring the voltage of each of the points (A), (B), and (C).

As an electrolyte generally used for anodization, a boric acid-basedformation solution, an oxalic acid-based formation solution, aphosphoric acid-based formation solution, or an adipic acid-basedformation solution can be used. For example, the phosphoric acid-basedformation solution is prepared by dissolving 1.5 g of ammonium phosphatein 1 L of water. The adipic acid-based formation solution is prepared bydissolving 1 g of ammonium adipate in 1 L of water. In evaluation ofthese electrolytic solutions by the above-described re-formation method,the points (A), (B), and (C) lie in ranges of 10 V to 100 V, 20 V to 180V, and 60 V to 200 V, respectively. With an acid formation solution suchas the oxalic acid-based formation solution, the point (A) appears at alow voltage, and the point (C) also appears at a relatively low voltage.On the other hand, with a neutral formation solution such as the adipicacid-based formation solution, the point (C) can be increased, but thepoint (A) is also disadvantageously increased.

When the above-described ionic liquid containing a fluorine anion, anionic liquid containing a sulfonic acid (—SO₃ ⁻) anion, or an ionicliquid containing a carboxylate (—COO⁻) anion is evaluated by there-formation evaluation method, for example, with a voltage of firstoxide film formation of 200 V, generally, the maximum current point (A)is in a range of 10 V to 25 V, and the minimum current point (B) is in arange of 30 V to 50 V. On the other hand, the current increase point (C)is in a range of 100 V to 200 V. These properties vary depending on thetype of the ionic liquid, particularly the type of the anion. Thefluorine anion-containing ionic liquid has the characteristic that thevoltage of the point (C) is particularly increased (160 V or more), andthe withstand voltage is excellent. In contrast, the ionic liquidcontaining a sulfonic acid (—SO₃ ⁻) anion and the ionic liquidcontaining a carboxylate (—COO⁻) anion show the point (C) at 60 V to 100V.

This fact significantly appears when an oxide film is first formed at200 V or less, for example, 50 V or 100 V. For example, with a voltageof first oxide film formation of 50 V, the ionic liquid containing asulfonic acid (—SO₃ ⁻) anion and the ionic liquid containing acarboxylate (—COO⁻) anion show the point (C) at 30 V to 60 V, while thefluorine anion-containing ionic liquid shows the point (C) at 80 V to170 V. On the other hand, with a voltage of first oxide film formationof 100 V, the ionic liquid containing a sulfonic acid (—SO₃ ⁻) anion andthe ionic liquid containing a carboxylate (—COO⁻) anion show the point(C) at 50 V to 80 V, while the fluorine anion-containing ionic liquidshows the point (C) at 120 V to 200 V.

This indicates that the ionic liquid exhibits an excellent anodizingproperty in a relatively low voltage region (i.e., the point (A) appearsat a low voltage), as compared with a general solution of an organicsalt electrolyte in an organic solvent, but the withstand voltage (i.e.,the point (C) at a low voltage) of an electrolyte in a high-voltageregion must be improved according to the type of the ionic liquid, forexample, the ionic liquid containing a sulfonic acid (—SO₃ ⁻) anion orthe ionic liquid containing a carboxylate (—COO⁻) anion.

As a result of further research for improving the properties of theionic liquids, it was found that the withstand voltage can be improvedby adding an additive, such as an ammonium salt, an amine salt, aquaternary ammonium salt, a tertiary amine, or an organic acid, to anionic liquid. An ionic liquid can sufficiently dissolve an ammoniumsalt, an amine salt, a quaternary ammonium salt, or an organic acid.Examples of the additive includes ammonium salt additives, such asammonium adipate; amine salt additives, such as triethylamine maleate;quaternary ammonium salt additives, such as quaternary ammonium maleateand quaternary ammonium phthalate; ammonium phosphate additives, such asammonium dihydrogen phosphate; ammonium borate; quaternary imidazoliumsalts; malic acid; and succinic acid. When such an additive is added tothe above-described zwitterion-type ionic liquid, the melting point canbe decreased, and thus the addition is effective to the object of thepresent invention.

In addition to the ionic liquid as a component, the electrolyte of thepresent invention may further contain a solute added for improving theperformance as an electrolyte. Since the solute added to the electrolyteof the present invention is always present in a dissolved state becausethe ionic liquid actually does not evaporate, and thus the anodizingproperty of the solute is added to the anodizing property of the ionicliquid, thereby further improving the performance as an electrolyte.Examples of such a solute include ammonium borate, ammonium phosphate,and ammonium adipate. This method is particularly effective to a case inwhich the anodizing ability of the ionic liquid is not so high. Also,the addition of the solute can control a physical property, and forexample, decrease the melting point of the ionic liquid as a componentdue to the freezing-point depression effect of the solute added.

The amount of the solute added to the ionic liquid can be arbitrarilyselected in a range which does not lose any one of the liquid propertiesof the ionic liquid. For example, when ammonium adipate is added to theionic liquid, the adding amount is preferably 1% by weight (simplyreferred to as “%” hereinafter) or more and lower than 50%, i.e., anammonium adipate/ionic liquid ratio of 1/1, for improving the anodizingperformance depending on the type of the ionic liquid. When ammoniumborate is added, the adding amount is preferably lower than 50%.Similarly, when ammonium phosphate is added, the adding amount ispreferably lower than 10%. The solubility of the solute in the ionicliquid is high, and a relatively large amount of the solute can bedissolved in the ionic liquid. This is of advantage to using the ionicliquid.

The ionic liquid containing an AlCl⁻, Cl⁻, or Br⁻ anion may dissolve anoxide film by corrosion and etching. However, the ionic liquidcontaining fluorine-containing anion molecules favorably has no adverseeffect, such as etching, on an oxide film. The degree of the anodizingability depends on the molecular structure, but, in particular, thefluorine anion-containing ionic liquid which is hydrophilic has theexcellent anodizing ability. The decision as to whether the ionic liquidis hydrophilic or hydrophobic is made on the criterion that when theionic liquid is completely mixed with the water added, the ionic liquidis decided as hydrophilic, and when the ionic liquid and the water addedare separated into two layers, the ionic liquid is decided ashydrophobic. The ionic liquid formed using BF₄ ⁻ as an anion isfrequently hydrophilic, and the ionic liquid formed by combining withimidazolium cations or pyridinium cations exhibits the excellentanodizing property.

Next, description will be made of the large practical advantage of theuse of the ionic liquid in forming an oxide film on a metal surface,i.e., the advantage that the ionic liquid actually does not evaporatebecause of its very low vapor pressure. As described above, theelectrolyte for anodization is generally used as an aqueous solution ora solution in an organic solvent, thereby causing the problem that wateror an organic solvent used as the solvent evaporates. In a state inwhich the solvent evaporates to leave the solid solute alone, theability of forming an oxide film on a metal surface is lost. Namely, aconventional electrolyte cannot be used in an environment in which asolvent evaporates. On the other hand, the method for forming an oxidefilm on a metal surface using the ionic liquid can be performed undersuch a condition that a general electrolyte cannot be used.

As an application utilizing the advantage of reaction of oxide filmformation on a metal surface using the ionic liquid, use of the ionicliquid as a capacitor electrolyte will be described. When the ionicliquid of the present invention is used as a capacitor electrolyte, afirst advantage is having the excellent anodizing property, and a secondadvantage is that the ionic liquid actually does not evaporate in anordinary use state because of its very low vapor pressure.

In an electrolytic solution-type conventional electrolytic capacitorusing an organic solvent, such as γ-butyrolactone, the electrolyticsolution used further contains a solute added to the organic solvent.When such an organic solvent evaporates in long-term use, the soluteadded becomes solid after evaporation of the organic solvent, and thusthe anodizing property which is the oxide film repairing ability cannotbe exhibited. However, when the solute added is liquid, the solutecomponent remains even after evaporation of the organic solvent, andthus the anodizing ability is not completely lost. Therefore, it isadvantageous as an application of the present invention that the ionicliquid of the invention is used for an electrolytic solution-typecapacitor.

The ionic liquid of the present invention is more preferably used for asolid electrolytic capacitor. In a solid electrolytic capacitor, onlythe anodizing property can be achieved to some extent by dissolving asolute, such as an ammonium salt, an amine salt, a quaternary ammoniumsalt, or an organic acid, in an organic solvent, and then adding theresultant solution to a conductive polymer or a TCNQ salt. However,there is the disadvantage that the effect of the addition is lost byevaporation of the solvent in long-term use.

When the electrolyte of the present invention is used for a solidcapacitor, the ionic liquid is particularly preferably added (combinedwith) to a conductive polymer electrolyte or a TCNQ salt electrolyte.This is because the excellent electron conductivity of the conductivepolymer electrolyte or TCNQ salt electrolyte is added to the excellentanodizing property of the ionic liquid, thereby realizing an idealcapacitor electrolyte.

First, use of the conductive polymer as the electrolyte will bedescribed. The conductive polymer is not particularly limited, butpolypyrrole, polythiophene, polyaniline, and derivatives thereof arepreferred. An example of the derivatives is polythiophene obtained froma 1,4-dioxythiophene monomer. As a method for synthesizing theconductive polymer, a chemical polymerization method, an electrolyticpolymerization method, and an organometallic chemical polycondensationmethod are used, and the chemical polymerization method and theelectrolytic polymerization method are particularly preferably used.

The electrolytic polymerization is a method in which for example, apyrrole monomer is dissolved in a solvent together with a supportingelectrolyte and then subjected to dehydrogenation polymerization byanodization. This method is capable of depositing polypyrrole as theconductive polymer on an anode. Since a polymer generally has a loweroxidation-reduction potential than that of a monomer, oxidation of apolymer skeleton further proceeds in the polymerization process, andanion of the supporting electrolyte are taken as a dopant in the polymerwith the oxidation. The mechanism of the electrolytic polymerization hasthe advantage that a conductive polymer is obtained without subsequentaddition of a dopant.

On the other hand, the chemical polymerization is a method in which araw material monomer, for example, pyrrole, is polymerized by oxidationdehydration in the presence of an appropriate oxidizing agent. As theoxidizing agent, a persulfate, hydrogen peroxide, or a transition metalsalt of iron, copper, manganese, or the like can be used. The conductivepolymer synthesized by the chemical polymerization contains the anion ofthe oxidization agent which is also taken as a dopant in the polymerduring the polymerization process, and thus the conductive polymer canbe obtained by one-stage reaction.

The chemical polymerization in the ionic liquid is particularlypreferred for the object of the present invention because the anion ofthe ionic liquid may be taken as a dopant in the conductive polymer.

In the present invention, the dopant of the conductive polymer used as acomponent of the electrolyte is selected in view of the influence on theconductivity and thermal stability of the conductive polymer. Preferredexamples of the dopant used in the present invention include4-fluroboric acid ions, p-toluenesulfonic acid ions,anthraquinone-2-sulfonic acid ions, triisopropylnaphthalenesulfonic acidions, polyvinylsulfonic acid ions, dodecylbenzenesulfonic acid ions,alkylsulfonic acid ions, n-propylphosphoric acid ions, and perchloricacid ions.

In order that the dopant is taken in the polymer by the electrolyticpolymerization method, the dopant may be dissolved in the form of asodium salt, an ester, an ammonium salt, or the like, such as sodiump-toluenesulfonate, sodium dodecylbenzenesulfonate, n-propylphosphate,tetra-n-butylammonium perchlorate, or the like, in a solvent, such aswater or a nonaqueous solvent (acetonitrile, dimethylformamide, or thelike), followed by the electrolytic polymerization in the resultantsolution.

In use as an electrolyte for an electrolytic capacitor, the electrolyteis disposed on the surface of an oxide film formed on a valve metal,such as aluminum, tantalum, or niobium. The metal functions as an anodeof an electrolytic capacitor and is used as an etched foil or a sinteredbody of a metal powder, for increasing the surface area. Therefore, whenthe conductive polymer is synthesized by the chemical polymerization,the spaces of the etched foil or the sintered powder must be filled withthe conductive polymer. On the other hand, when the conductive polymeris synthesized by the electrolytic polymerization, an oxide film on avalve metal is a dielectric, and thus a conductive film must bepreviously formed on the dielectric for making the film conductive,followed by the electrolytic polymerization with the current or voltageapplied from a power supply source. As the conductive film used for thispurpose, a conductive polymer synthesized by the chemicalpolymerization, pyrolytic manganese dioxide, or the like can be used.

Next, description will be made of a method for compounding the ionicliquid and the conductive polymer to form the electrolyte of the presentinvention.

The simplest compounding method includes forming the conductive polymeron an oxide film on a valve metal by a known method, immersing theconductive film in the ionic liquid, and then puling up the conductivefilm from the ionic liquid. The ionic liquid may contain a solute. Informing an electrolytic capacitor, a cathode forming step, an electrodemounting step, an armoring step, and an aging step may be subsequentlyperformed. When an aluminum case is used, for example, for a coiledelectrolytic capacitor, the ionic liquid is preferably added into thealuminum case.

The amount of the ionic liquid added is selected in a range in which thesatisfactory anodizing property is exhibited, and the electronconductivity of the conductive polymer is not impaired. From theviewpoint that the electron conductivity is not impaired, the amount ofthe ionic liquid added is preferably a weight ratio of less than 1/10 tothe conductive polymer. From the viewpoint of the satisfactory anodizingproperty, the amount of the ionic liquid added is preferably a weightratio of 1/10000 or more and more preferably 1/1000 or more to theconductive polymer. In other words, the weight ratio (ionicliquid/conductive polymer) of the ionic liquid to the conductive polymerin the electrolyte of the present invention is preferably in a range of1/10000 to less than 1/10 and more preferably 1/1000 to less than 1/10.

The necessary amount of the ionic liquid of the present invention may besignificantly smaller than that of an electrolytic capacitor using anelectrolyte composed of a conductive polymer and an organic onium salt,for example, as disclosed in Patent Document 3. In Patent Document 3, inorder to improve the withstand voltage, the ratio between the conductivepolymer (A) and the organic onium salt (B) is preferably (A):(B)=1:0.1to 5 and more preferably (A):(B)=1:0.2 to 2. However, in the presentinvention, as described above, the amount of the ionic liquid added isless than 10% relative to the conductive polymer, and the high electricconductivity of the conductive polymer is not impaired by adding theionic liquid, thereby realizing a capacitor having excellent impedanceproperties.

A second compounding method uses the ionic liquid as a solvent forsynthesizing the conductive polymer by the electrolytic polymerizationor chemical polymerization so that the solvent used is positively left,for example, after the step of forming an electrolyte for anelectrolytic capacitor. In this case, as described above, the weightratio (ionic liquid/conductive polymer) of the ionic liquid to theconductive polymer is preferably in a range of 1/10000 to less than 1/10and more preferably 1/1000 to less than 1/10.

In this method, it is more preferable as the compounding method that theanion type of the ionic liquid is the same as that of the dopant of theconductive polymer. There has been not known an example in whichelectrolytic polymerization reaction and doping of the conductivepolymer are simultaneously performed with the same anion type, the ionicliquid is used as a solvent in synthesizing the conductive polymer bychemical polymerization which is capable of producing an electrolytehaving both excellent electron conductivity and excellent ionconductivity, and then the ionic liquid is positively left after thepolymerization and is added to the produced conductive polymer.

Next, use of the TCNQ salt as an electrolyte will be described. Althoughthe TCNQ salt is not particularly limited, a TCNQ complex salt using anammonium cation is preferably used. In particular, a TCNQ complex saltcontaining a nitrogen-containing heterocyclic compound having an alkylsubstituent at the N position and used as a donor and TCNQ used as anacceptor is preferred for the object of the present invention. Examplesof the nitrogen-containing heterocyclic compound include pyridine,pyridine derivatives such as lutidine, quinoline, quinoline derivativessuch as isoquinoline, acridine, phenazine, and phenanthroline. Exampleof the alkyl substituent at the N position include butyl, amyl, hexyl,and phenethyl. As the electrolyte, these TCNQ salts are used alone or asa mixture of two or more, and an additive such as a glucose polymer maybe added according to demand. The TCNQ salt is synthesized by dissolvingTCNQ in a solvent such as purified and dehydrated acetonitrile, addingan ammonium salt (for example, N-n-butylisoquinolinium iodide or thelike) to the solution, and separating and filtering off the precipitatedTCNQ salt.

Examples of such a TCNQ salt include N-n-butylisoquinolinium (TCNQ)₂salt, N-isoamylisoquinolinium (TCNQ)₂ salt, N,N-pentamethylene(lutidine)₂(TCNQ)₄ salt, N-phenethyl-lutidine (TCNQ)₂ salt, and mixturesof these TCNQ salts. The reasons why these TCNQ salts are particularlypreferably used are that the salts have relatively high conductivity andthat the salts have the peculiar property of being molten by heating.General TCNQ salts are decomposed or sublimed by heating, not molten byheating. An electrolyte for an electrolytic capacitor is disposed on thesurface of a dielectric oxide film formed on a valve metal, such asaluminum, tantalum, or niobium. The metal functions as an anode of theelectrolytic capacitor and is used as an etched foil or a sintered bodyof a metal powder, for increasing the surface area. Therefore, the poresof the etched foil or the spaces of the sintered powder must be filledwith the TCNQ salt. The TCNQ salt property of being molten is excellentfor a method used for melting and filling the TCNQ salt in an etchedaluminum electrode or a sintered tantalum electrode.

The amount of the ionic liquid added is selected in a range in which thesatisfactory anodizing property is exhibited, and the electronconductivity of the TCNQ salt is not impaired. From the viewpoint thatthe electron conductivity is not impaired, the amount of the ionicliquid added is preferably a weight ratio of less than 1/2, morepreferably 1/5 or less, and most preferably 1/10 or less to the TCNQsalt. From the viewpoint of the satisfactory anodizing property, theamount of the ionic liquid added is preferably a weight ratio of 1/10000or more and more preferably 1/1000 or more to the TCNQ salt. In otherwords, the weight ratio (ionic liquid/TCNQ salt) of the ionic liquid tothe TCNQ salt in the electrolyte of the present invention is preferablyin a range of 1/10000 to less than 1/2 and more preferably 1/10000 to1/5 to, and most preferably 1/1000 to 1/10.

Next, a method for compounding the ionic liquid and the TCNQ salt forforming the electrolyte of the present invention will be described. Thecombining method is capable of obtaining an electrolyte having excellentelectron conductivity and the excellent anodizing property.

The simplest compounding method includes forming the TCNQ salt on anoxide film on a valve metal by a known method, immersing the TCNQ saltin the ionic liquid, and then pulling up the TCNQ salt therefrom. Inorder to form an electrolytic capacitor, a cathode forming step, anelectrode mounting step, an armoring step, and an aging step may besubsequently performed.

In a coiled capacitor using an aluminum case, the ionic liquid and theTCNQ salt are added into the aluminum case and heat-molten, and then acapacitor element coiled together with Manila hemp paper and includingan anode and a cathode is inserted in the aluminum case and impregnatedwith the ionic liquid and the TCNQ salt. When an electrolytic capacitoris formed, a sealing step and an aging step may be subsequentlyperformed. For a capacitor which is not a coiled type, the TCNQ salt maybe formed on an oxide film on a valve metal by a known method, immersedin the ionic liquid, and then pulled up therefrom. When an electrolyticcapacitor is formed, a cathode mounting step, an armoring step, and anaging step may be subsequently performed.

The present invention will be described in further detail below withreference to examples.

EXAMPLES

(Ionic Liquid)

First, the ionic liquid used as a component of the electrolyte of thepresent invention is described. Hereinafter, when a synthesized materialwas used, a synthesis method therefor is described, while when acommercial available material was used, a synthesis method therefor isnot described. The molecular formulae, physical properties, andabbreviations (ILS-1 to ILS-23) of the ionic liquids used are givenbelow. In the formulae, Im represents imidazolium, and Py representspyridinium.

(ILS-1) (1-C₂H₅-3-C₂H₅-Im)⁺(p-CH₃—C₆H₄SO₃)⁻

In a 200-ml dry round-bottom flask, 4.02 g (41.7 mmol) ofN-ethylimidazole and 20 ml of DMF were charged, and the resultantmixture was sufficiently stirred. Then, 8.35 g (41.7 mmol) of ethylp-toluenesulfonate was rapidly added to the flask under cooling withice. After the addition, the mixture was further stirred for 23 hours.The reaction solution was added dropwise to 200 ml of ice-cooled ether.The ether was decanted to recover 8.1 g of a yellow liquid. The yieldwas 65.5%. The recovered liquid was identified by its ¹H-NMR spectrum.The obtained product had a glass transition temperature (Tg) of −59.5°C.

[Spectral data]: 500 MHz, ¹H-NMR (DMSO-d₆) σ=1.35 (triplet, J=5 Hz, 3H),2.23 (singlet, 3H), 4.15 (quarlet, J=5 Hz, 2H), 7.06 (doublet, J=5 Hz,2H), 7.44 (doublet, J=5 Hz, 2H), 7.74 (singlet, 2H), 9.04 (singlet, 3H)

(ILS-2) (1-CH₃-3-C₂H₅-Im)⁺(p-CH₃—C₆H₄SO₃)⁻

1-Methyl-3-ethylimidazolium p-toluenesulfonate was synthesized by thesame method as described above. The product was a yellow liquid, and theyield was 74.4%. The recovered liquid was identified by its ¹H-NMRspectrum. The obtained product had a glass transition temperature (Tg)of −85.7° C. and a melting point of −12.7° C.

[Spectral data]: 500 MHz, ¹H-NMR (DMSO-d₆) σ=1.33 (triplet, J=5 Hz, 3H),2.22 (singlet, 3H), 3.77 (singlet, 3H), 4.12 (quarlet, J=5 Hz, 2H), 7.06(doublet, J=5 Hz, 2H), 7.44 (doublet, J=5 Hz, 2H), 7.65 (singlet, 2H),7.72 (singlet, 2H), 9.08 (singlet, 3H)

(ILS-3) (1-nC₄H₉-3-C₂H₅-Im)⁺(p-CH₃—C₆H₄SO₃)⁻

1-Butyl-3-ethylimidazolium p-toluenesulfonate was synthesized by thesame method as described above. The product was a yellow liquid and hada glass transition temperature (Tg) of −73.8° C.

(ILS-4) (1-C₂H₅-Im)⁺(C₆H₅SO₃)⁻

1-Ethylimidazolium benzenesulfonate was synthesized by the same methodas described above. The product was a colorless transparent liquid andhad a glass transition temperature of −65.1° C. and a melting point of−9.5° C.

(1LS-5) (1-C₂H₅-Im)⁺(CH₃COO)⁻

First, 6 ml of 99.7% acetic acid was added to 10 g of N-ethylimidazole,and the resultant mixture was stirred for 12 hours at a temperature keptat 0° C. The resultant reaction product was added dropwise to 1000 ml ofdiethyl ether under stirring. The diethyl ether was distilled off atroom temperature, and the residue was dried under vacuum to precipitatecrystals. The precipitated crystals were recovered to obtain 15.9 g ofN-ethylimidazolium acetate. The product had a glass transitiontemperature of −51.7° C.

(ILS-6) (1-nC₄H₉-2-CH₃-3-CH₃-Im)⁺(C₂H₅OC₂H₄OSO₃)⁻: brown liquid, meltingpoint −4.2° C.

(ILS-7) (1-nC₄H₉-3-CH₃-Im)⁺(CHF₂CF₂CF₂CF₂CH₂SO₃)⁻: yellow liquid,melting point −62° C.

(ILS-8) (1-C₂H₅-Im)⁺(BF₄)⁻: colorless liquid, melting point −53.3° C.

(ILS-9) (1-C₂H₅-3-CH₃-Im)⁺((CF₃SO₂)₂N)⁻: colorless liquid, melting point−18.2° C.

(ILS-10) (1-nC₆H₁₃-Py)⁺((CF₃SO₂)₂N)⁻: yellow liquid.

(ILS-11) (1-CH₃-2-CH₃-3-C₂H₅-4-C₂H₄OC₂H₄OCH₃N)⁺((CF₃SO₂)₂N)⁻: colorlessliquid.

(ILS-12) (1-CH₃-3-C₂H₅-Im)⁺((CF₃SO₂)₃C)⁻: yellow liquid.

(ILS-13) (1-C₂H₅-Im)⁺(CH₃CH₂CH₂CH₂SO₃)⁻

First, 5.30 g (55.1 mmol) of N-ethylimidazole was dissolved in 50 ml ofacetone. Next, 7.61 g (55.9 ml) of propane sultone was dissolved in 100ml of acetone, and the resultant solution was added dropwise to theN-ethylimidazole acetone solution at room temperature, followed byreaction under further stirring for 91 hours at room temperature. Theresultant reaction mixture was filtered by suction using a Nutscheaspirator provided with a glass filter. The product filtered off on theglass filter was sufficiently washed with excess acetone and then driedunder vacuum to obtain 1.42 g of the product. The yield was 11.1%. Theproduct was identified as 1-(N-ethylimidazolio)butane-4-sulfonate by its¹H-NMR spectrum. As a result of measurement by differential scanningcalorimetry (DSC), the melting point was −10° C.

[Spectral data]: 500 MHz, 1H-NMR (DMSO-d₆) σ=1.36 (triplet, 3H), 1.48(triplet, 2H), 1.84 (triplet, 2H), 2.36 (triplet, 2H), 4.13 (multiplet,4H), 7.77 (d.d., 2H), 9.20 (singlet, 1H)

(ILS-14) (1-C₂H₅-Im)⁺(C₆H₅SO₃)⁻

First, 4.02 g (41.7 mmol) of N-ethylimidazole was dissolved in 50 ml ofethanol. Next, 8.35 g (41.7 mmol) of p-toluenesulfonic acid monohydratewas rapidly added to the N-ethylimidazole ethanol solution underice-cooling, followed by stirring for 23 hours. The ethanol wasdistilled off with an evaporator, and the residual reaction solution wasadded dropwise to 200 ml of ether cooled with dry ice. The resultantmixture was rapidly filtered by suction using a Nutsche aspiratorprovided with a glass filter to recover 8.10 g of the product on theglass filter. The yield was 65.5%. The product was identified as1-ethyl-imidazolium-p-toluenesulfonate by its ¹H-NMR spectrum. Theobtained imidazolium salt had a glass transition temperature (Tg) of4.3° C.

[Spectral data]: 500 MHz, 1H-NMR (DMSO-d₆, σ) σ=1.35 (triplet, J=5 Hz,3H), 2.23 (singlet, 3H), 4.15 (quarlet, J=5 Hz, 2H), 7.06 (doublet, J=5Hz, 2H), 7.44 (doublet, J=5 Hz, 2H), 7.74 (singlet, 2H), 9.04 (singlet,1H)

(ILS-15) (1-nC₄H₉-Im)⁺(p-CH₃—C₆H₄SO₃)⁻

First, 3.80 g (30.6 mmol) of N-butylimidazole was dissolved in 20 ml ofDMF (dimethylformamide). Next, 5.20 g (30.6 mmol) of p-toluenesulfonicacid monohydrate was rapidly added to the N-butylimidazole DMF solutionunder ice-cooling, followed by stirring for 23 hours. The reactionsolution was added dropwise to 200 ml of ether cooled with dry ice. Theresultant mixture was rapidly filtered by suction using a Nutscheaspirator provided with a glass filter to recover 6.40 g of a whitesolid on the glass filter. The yield was 70.6%. The recovered productwas identified as 1-butyl-imidazolium-p-toluenesulfonate by its ¹H-NMRspectrum. The obtained imidazolium salt had a glass transitiontemperature (Tg) of −38.4° C. and a crystallization temperature (Tc) of2.6° C.

[Spectral data]: 500 MHz, 1H-NMR (DMSO-d₆) σ=0.84 (triplet, J=5 Hz, 3H),1.16 (multiplet, 2H), 1.71 (multiplet, 2H), 2.23 (singlet, 3H), 4.11(trilet, J=5 Hz, 2H), 7.07 (doublet, J=5 Hz, 2H), 7.44 (doublet, J=5 Hz,2H), 7.60 (singlet, 1H), 7.71 (singlet, 1H), 9.04 (singlet, 3H)

(ILS-16) (1-CH₂═CH-Im)⁺(CH₃SO₃)⁻

First, 7 ml of methanesulfonic acid was added to 10 g ofN-vinylimidazole, and the resultant mixture was stirred for 3 hours at atemperature kept at 0° C. and then added dropwise to diethyl ethercooled with dry ice. The resultant mixture was rapidly filtered bysuction using a Nutsche aspirator provided with a glass filter torecover crystals on the glass filter. The crystals were dried to obtain19.2 g of N-vinylimidazolium methanesulfonate. The yield was 95%, andthe melting point was 5° C.

(ILS-17) N-Vinylimidazolium Molten Salt Polymer

First, 1.0 g of the ILS-16 was dissolved in 10 ml of methanol, andazobisisobutyronitrile was added as a polymerization initiator to theresultant solution at a molar ratio of 1% relative to the vinyl monomerunit of the ILS-16. Then, radical polymerization was performed at atemperature of 65° C. for 3 hours to obtain N-vinylimidazolium moltensalt polymer (ILS-17).

(ILS-18) (1-nC₄H₉-3-CH₃-Im)⁺(BF₄)⁻

1-Butyl-3-methylimidazolium tetrafluoroborate (mp −71° C.), manufacturedby Kanto Kagaku Co., Ltd.

(ILS-19) (1-nC₄H₉-Py)⁺(BF₄)⁻

1-Butylpyridinium tetrafluoroborate (mp −88° C.), manufactured by KantoKagaku Co., Ltd.

(ILS-20) (1-nC₆H₁₃-3-CH₃-Im)⁺(PF₆)⁻

1-Hexyl-3-methylimidazolium hexafluorophosphate (mp −73° C.),manufactured by Kanto Kagaku Co., Ltd.

(ILS-21) (1-C₂H₅-3-CH₃-Im)⁺(CF₃SO₃)⁻

1-Ethyl-3-methylimidozolium trifluoromethanesulfonate (mp −9° C.),manufactured by Kanto Kagaku Co., Ltd.

(ILS-22) (1-nC₆H₁₃-3-CH₃-Im)⁺(Br)⁻

1-Hexyl-3-methylimidazolium bromide (mp −52° C.), manufactured by KantoKagaku Co., Ltd.

(ILS-23) (1-nC₆H₁₃-3-CH₃-Im)⁺(Cl)⁻

1-Hexyl-3-methylimidazolium chloride (mp −85° C.), manufactured by KantoKagaku Co., Ltd.

(ILS-24) (1-C₂H₅-3-CH₃-Im)⁺(Cl)⁻

1-Ethyl-3-methylimidazolium chloride, manufactured by Kanto Kagaku Co.,Ltd.

(ILS-25) (1-C₂H₅-3-CH₃-Im)⁺(Br)⁻

1-Ethyl-3-methylimidazolium bromide, manufactured by Kanto Kagaku Co.,Ltd.

(Synthesis of TCNQ Salt)

Synthesis examples of the TCNQ salt used as a component of theelectrolyte of the present invention will be described.

(Salt A) N-n-Butylisoquinolinium (TCNQ)₂ Salt

To a flask provided with a reflux condenser, commercial n-butyl iodide(20 mmol) and isoquinoline (20 mmol) were added, followed by heating to80° C. Since a yellow oily product was separated from a liquid phase,heating was stopped when the product started to be generated, and thereaction was controlled to slowly proceed using hot water (about 40°C.). The reaction proceeded about 100%, and thus the reaction wasterminated when the whole reaction solution was turned to an oily state.When the heating was stopped, the product was immediately crystallized(solidified). The product was washed with ethyl ether and then purifiedby recrystallization with methanol.

The n-butylisoquinoline iodide (25 mmol) obtained by the above-describedmethod and TCNQ (30 mmol) were dissolved in 30 ml of acetonitrile and 60ml of acetonitrile, respectively, under heating, and both solutions weremixed while being gently boiled. After mixing, the resultant mixture washeated for 1 hour under reflux to complete the reaction. After thecompletion of the reaction, the mixture was allowed to stand at roomtemperature for 1 hour and cooled at 5° C. overnight, and the produceddark purple crystals were filtered off. The resultant crystals werewashed with a small amount of cooled acetonitrile and further with ethylether. The resultant salt had an electric conductivity of 3.4 Ωcm and amelting point of 210° C., and the yield was 80%.

(Salt B) N-Isoamylisoquinolinium (TCNQ)₂ Salt

An N-isoamylisoquinolium (TCNQ)₂ salt was synthesized by the same methodas that for salt (A) except that n-isoamyl iodide was used in place ofn-butyl iodide. The resultant salt had an electric conductivity of 4.2Ωcm and a melting point of 213° C., and the yield was 78%.

(Evaluation of Anodizing Ability)

An aluminum anodization experiment using an ionic liquid will bedescribed. The anodizing property was evaluated as follows: A filmformed on an aluminum plate with a purity of 99.9% by anodization at 200V using an adipic acid solution was partially broken with boiling water.The film sample was subjected to re-formation (re-anodization) in anionic liquid to measure changes in a current flowing with a voltageapplied.

Example 1

An aluminum wire (1.5 mm in diameter) with a purity of 99.99% was dippedin a mixture containing 70% HNO₃ (15 parts) and 85% H₃PO₄ (85 parts) for2 minutes, and then washed with pure water. Next, the wire was etchedwith a 1N NaOH solution for 10 minutes, washed with pure water, dippedin acetone, and then dried.

Next, the aluminum wire was subjected to formation treatment in anaqueous solution (1 g/L) of adipic acid. The formation treatment wasperformed with a constant current of 10 mA/cm², and after the voltagereached 200 V, the wire was maintained at a constant voltage of 200 Vfor 10 minutes. After the formation, the formed film was treated withboiling water for 3 minutes with 100 V and a DC current applied so thatthe Al (aluminum) side was a positive pole. The formed film waspartially broken by the treatment.

The formed film treated as described above was immersed in an ionicliquid, and changes in the current value with increases in the voltageat a rate of 1 V/sec at room temperature were measured. FIG. 2 shows thechanges in the current using the ILS-1.

The current first flows through the broken portion of the oxide film(region 1). However, when the electrolytic solution has the anodizingproperty, a new oxide film is formed in the broken portion by the filmrepairing ability of the electrolytic solution, and thus the currentdecreases beyond the maximum (point A: near 15 V) (region 2). Theminimum current value (point B: near 40 V) is at the end point ofrepair, and a linear current increase region based on ion conductivitythen appears (region 3), in which the current increases in proportion toincreases in the voltage. However, when the voltage is furtherincreased, the current deviates from the linear relation at a voltage(point C: near 80 to 100 V) and starts to flow (region 4). This voltageindicates the actual withstand voltage of the electrolytic solution. Ofcourse, when the electrolytic solution has no anodizing ability, onlythe region 1 appears, and the current continuously flows to lead to thebreakage of an oxide film. It was confirmed from these results that theILS-1 has the excellent anodizing property.

Comparative Example 1

The same re-formation (re-anodization) experiment was carried out usingan aqueous adipic acid solution. The results of the experiment are shownin FIG. 3. The aqueous adipic acid solution was prepared by dissolving 1g of ammonium adipate in 1 L of distilled water and had a conductivityof 400 Ωcm at 70° C. and a pH 6.8.

In use of adipic acid, the maximum current is shown near 45 V (point A)and then decreases to the minimum near 120 V (point B), and the currentthen starts to increase at a voltage of 180 V (point C).

In comparison between the formation characteristics of theabove-described ILS-1 and adipic acid, it is found that the ILS-1 is anexcellent material exhibiting the anodizing property (re-formation) at alow voltage, but, in the ILS-1, a withstand voltage of about 80 to 100 Vcan be realized even with the film formed with 200 V. However, in adipicacid, an excellent withstand voltage property up to 180 V is exhibited,but the re-formation ability is not exhibited in a low voltage region of40 V or less. Therefore, adipic acid is found to be disadvantageous inthat it cannot be applied to repair in such a low voltage region.

Examples 2 to 12

The same experiment as in Example 1 was carried out using each of theILS-2 to ILS-12. The results of the experiments are shown in Table 1. Inthe table, a blank (marked with “−”) indicates that a definite voltagevalue was not observed. TABLE 1 Evaluation of anodizing ability(re-formation experiment): influence of type of ionic liquid ExperimentNo. Ionic liquid Point A (V) Point B (V) Point C (V) Example 1 ILS-1 1540 80-100 Example 2 ILS-2 15 40-50 120 Example 3 ILS-3 15 40-50 120Example 4 ILS-4 15 40-50 100 Example 5 ILS-5 10 30-40 80 Example 6 ILS-6— 40-50 90 Example 7 ILS-7 — 50-70 170 Example 8 ILS-8 15 40-90 160Example 9 ILS-9 20 50 ≧200 Example 10 ILS-10 30 50-70 ≧200 Example 11ILS-11 30 50 ≧200 Example 12 ILS-12 30 70 180 Comparative 45 120  180Example 1 Comparative ILS-24 — — — (20 or less) Example 2 ComparativeILS-25 — — — (20 or less) Example 3

Table 1 shows the voltages of the points A, B, and C. In the table, ablank (marked with “−”) indicates that a definite voltage value was notobserved.

In the table, each of the ILS-2 to ILS-12 is ILS or fluorineanion-containing ILS according to the present invention. The resultsindicate that any one of the ILSs has the excellent anodizing ability.However, the results also indicate that the voltages of the points A, B,and C vary depending on the type of the ILS, and the ILSs have differentperformances. Any one of the fluorine anion-containing ILSs has a highwithstand voltage (point C). The ILSs each containing an atomic groupanion containing a sulfonic acid anion (—SO₃ ⁻) or carboxylate anion(—COO⁻) are inferior in the withstand voltage property to the fluorineanion-containing ILSs but have the excellent anodizing ability (i.e.,low points A and B). It is further found that the anodizing property islittle affected by the type of the cation component, for example,imidazolium, pyridinium, or ammonium.

Comparative Examples 2 and 3

The same experiment as in Example 1 was carried out using each of twotypes of ionic liquids, i.e., 1-ethyl-3-methylimidazolium chloride(ILS-24) and 1-ethyl-3-methylimidazolium bromide (ILS-25). As a result,it was found that when an anion is a chlorine or bromine anion, thepoints A, B, and C do not obviously appear, and the withstand voltage isextremely low (i.e., the point C at 20 V or less). This is possibly dueto the fact that a metal oxide film is etched with a chlorine or bromineanion component. Therefore, it is thought that a chlorine- orbromine-containing ionic liquid is unsuitable for the object of thepresent invention.

Examples 13 to 22

The same experiment as in Example 1 was carried out except that thefirst formation voltage was 50 V or 100 V. The results are shown inTable 2. In the table, a blank (marked with “−”) indicates that adefinite voltage value was not observed. TABLE 2 Evaluation of anodizingability (re-formation experiment): influence of formation voltage IonicFormation Point A Point B Point C liquid voltage (V) (V) (V) (V) Example13 ILS-2 50 15 20 60 Example 14 ILS-2 100 15 30 85 Example 15 ILS-3 5015 20 55 Example 16 ILS-3 100 15 25 80 Example 17 ILS-7 50 — 50 80Example 18 ILS-7 100 — 60 140-150 Example 19 ILS-8 50 15 — 160 Example20 ILS-8 100 15 — 180-200 Example 21 ILS-10 50 20 40-120 160 Example 22ILS-10 100 20 40-120 200

The results show that in 50-V formation, the ILSs (ILS-2 and ILS-3) eachcontaining an atomic group anion including a sulfonic acid anion (—SO₃⁻) have breakdown voltages of 55 to 60 V (point C), and the fluorineanion-containing ILSs (ILS-7, ILS-8, and ILS-10) have breakdown voltagesof 120 V to 160 V. The ILS-7 contains a fluorine-containing sulfonicacid anion, but it is close to the ILS-8 and ILS-10 from the viewpointof the withstand voltage. These results indicate that a breakdownvoltage of 50 V or more is obtained with a sample formed with 50 V. Thismeans that each of these ILSs has the same breakdown voltage even withan aluminum electrode not subjected to initial formation. It is thusfound that the ILSs have the excellent anodizing ability.

It is also found that in 100-V formation, the breakdown voltages of theILS-2 and ILS-3 are 85 V and 80 V, respectively, which are lower thanthe formation voltage. On the other hand, the breakdown voltages of theILS-7, ILS-8, and ILS-10 are 150 V, 180 V, and 200 V, respectively. Itis thus found that the fluorine anion-containing anionic liquids havethe excellent withstand voltage property.

Examples 23 to 27

The same re-formation (anodization) experiment as described above wascarried out using an electrolytic solution prepared by dissolving 10% byweight of adipic acid in an ionic liquid. The results are shown in Table3. In the table, a blank (marked with “−”) indicates that a definitevoltage value was not observed. TABLE 3 Evaluation of anodizing ability(re-formation experiment): influence of addition of adipic acid Point AIonic liquid (V) Point B (V) Point C (V) Example 23 ILS-1 + adipic acid15 40 170 Example 24 ILS-2 + adipic acid 15 40 180 Example 25 ILS-3 +adipic acid 15 40 190 Example 26 ILS-7 + adipic acid 15 — 200 Example 27ILS-10 + adipic acid 30 — ≧200

In a system prepared by adding 10% by weight of adipic acid to an ionicliquid, the point C can be increased (i.e., the withstand voltage of theelectrolyte can be improved) as compared with a system including only anionic liquid. This effect is not significant with the fluorineanion-containing ionic liquids (ILS-7 and ILS-10) originally having theexcellent withstand voltage property, but the effect significantlyappears with the ionic liquids (ILS-1, ILS-2, and ILS-3) each includingan atomic group containing a sulfonic acid anion (—SO₃—). Such acomposite ionic liquid does not lose the characteristic that theanodizing ability can be exhibited from a lower voltage region than thatof an aqueous adipic acid electrolytic solution. Namely, the compositeelectrolyte has the excellent film repairing ability from a low voltageregion and exhibits the excellent withstand voltage property in a highvoltage region. This method can be widely applied to ionic liquids, forcontrolling the oxide film formation properties thereof.

Examples 28 to 44

An electrolytic capacitor was experimentally produced using a conductivepolymer formed on an aluminum oxide film by electrolytic polymerization,and each of the ionic liquids was added to the electrolytic capacitor tomeasure the capacitor characteristics.

Namely, an aluminum foil (aluminum etched foil) of 7 mm in length and 10mm in width was immersed in a 3% aqueous solution of ammonium adipate,the aluminum foil being provided with an anode lead and having poreswhich were formed in the surface by etching. Then, anodization wasperformed at 70° C. with a voltage of 70 V applied to form an oxide filmas a dielectric film on the surface of the aluminum foil. Next, thealuminum foil was immersed in a 30% aqueous solution of manganesenitrate, naturally dried, and then subjected to pyrolysis at 300° C. for30 minutes to form a conductive layer including a manganese oxide layeron the dielectric film.

Next, a polypyrrole layer was formed on the foil by electrolyticpolymerization. FIG. 4 is a conceptual view of the apparatus used. Anelectrolytic solution (6) used for polymerization contained pyrrole (0.5M), a 30% alcohol solution of sodium triisopropylnaphthalenesulfonate(0.1 M), and water. As shown in FIG. 4, an aluminum foil (2) wasdisposed in an electrolytic polymerization solution, and apolymerization initiation electrode (1) was brought near to a manganesedioxide conductive layer (4). Then, a constant voltage of 1.5 V wasapplied across the polymerization initiation electrode (1) and a cathode(7) for 50 minutes to perform electrolytic polymerization reaction. As aresult, an electrolytically polymerized polypyrrole layer (5) was formedon the conductive layer.

Then, the aluminum foil was washed with water, dried, immersed in amethanol solution of each ionic liquid, and then dried to removemethanol. By the above-described method, the ionic liquid was added tothe electrolytically polymerized polypyrrole layer to obtain anelectrolyte of the present invention. The amount of each ionic liquidadded was 0.5 to 5% by weight of the conductive polymer. Next, a carbonlayer and a silver paste layer were provided on the electrolyte of thepresent invention to prepare a capacitor. The resultant capacitor of thepresent invention was aged at 20 V for 1 hour, and then the initialcapacitance, tan δ, impedance (120 Hz), and withstand voltage (V) weremeasured.

Table 4 shows the characteristics of the resultant capacitors. Theinitial capacitance, tan δ, and impedance were not so different fromthose of Comparative Example 4 in which the ionic liquid was not added,but the withstand voltages were significantly improved. It was thusfound that improvement in the withstand voltage of an electrolyticcapacitor can be realized by applying the electrolyte of the presentinvention to the electrolytic capacitor, the electrolyte being preparedby adding an ionic liquid to a conductive polymer. TABLE 4 Initialproperties of capacitor: aluminum/oxide film/(polypyrrole + ionicliquid) system Withstand Experiment Ionic liquid Capacitance tanδImpedance voltage No. added (μF) (%) (mΩ) (V) Example 28 ILS-1 4.6 1.295 28 Example 29 ILS-2 4.5 1.2 95 27 Example 30 ILS-3 4.8 1.2 90 26Example 31 ILS-4 4.6 1.2 92 25 Example 32 ILS-5 4.8 1.1 92 25 Example 33ILS-6 4.7 1.3 95 28 Example 34 ILS-7 4.7 1.2 95 38 Example 35 ILS-8 4.61.2 98 32 Example 36 ILS-9 4.7 1.3 98 35 Example 37 ILS-10 4.6 1.3 99 34Example 38 ILS-11 4.7 1.4 102 34 Example 39 ILS-12 4.5 1.4 98 38 Example40 ILS-13 4.7 1.2 95 28 Example 41 ILS-14 4.8 1.1 88 25 Example 42ILS-15 4.7 1.1 90 24 Example 43 ILS-16 4.8 1.2 92 24 Example 44 ILS-174.6 1.1 98 28 Comparative No 4.8 1.1 90 16 Example 4

In particular, significant improvement in the withstand voltage wasobserved with the fluorine anion-containing ionic liquids (ILS-7 to 12).

Examples 45 to 48

A capacitor was prepared by the same method as in Examples 28 to 44except that in preparing a film by electrolytic polymerization, anelectrolytic solution having a composition including methoxyphenol (0.15M), pyrrole (0.5 M), an alcohol solution of sodiumtriisopropylnaphthalenesulfonate (0.1 M), and water was used in place ofthe electrolytic solution including pyrrole (0.5 M), a 30% alcoholsolution of sodium triisopropylnaphthalenesulfonate (0.1 M), and water.The characteristics of the resultant capacitors are shown in Table 5.TABLE 5 Initial properties of capacitor: aluminum/oxidefilm/(polypyrrole + ionic liquid) system Withstand Experiment Ionicliquid Capacitance tanδ Impedance voltage No. added (μF) (%) (mΩ) (V)Example 45 ILS-18 4.8 1.2 93 25 Example 46 ILS-19 4.6 1.2 88 26 Example47 ILS-20 4.6 1.2 92 23 Example 48 ILS-21 4.7 1.3 100 17 Comparative No4.8 1.1 90 16 Example 4 Comparative ILS-22 4.6 2.4 890 0.4 Example 5Comparative ILS-23 4.6 3.1 900 0.4 Example 6

With ILS-18 to 20, the initial capacitance, tan δ, and impedance werenot so different from those of Comparative Example 4 in which the ionicliquid was not added, but the withstand voltages were significantlyimproved. It was thus found that improvement in the withstand voltage ofa capacitor can be realized by adding an ionic liquid. While withILS-21, substantially no influence on the capacitor characteristics wasobserved, and the withstand voltage was not greatly improved. This ispossibly due to the fact that ILS-21 has substantially no anodizingability or the very low anodizing ability.

Comparative Example 5 and 6

An aluminum electrolytic capacitor was experimentally produced byelectrolytic polymerization according to the same procedures as inExample 45. Each of the ionic liquids ILS-22 and ILS-23 was added to theresultant electrolytic capacitor, and the capacitor characteristics weremeasured. The characteristics of the capacitors are shown in Table 1. Itwas found that when an ionic liquid containing chlorine or bromine isadded, the capacitor characteristics are significantly degraded.

Examples 49 to 65

A conductive polymer electrolytic capacitor was produced using aconductive polymer formed on a tantalum oxide film by chemicalpolymerization. Each of the ionic liquids was added to the resultantcapacitor, and the capacitor characteristics were measured.

Namely, a rectangular parallelepiped tantalum sintered compact (2 mm inlength, 1.5 mm in height, and 1 mm in width) provided with an anode leadwas subjected to anodization at 85° C. for 60 minutes with a voltage of33.9 V applied in a 0.05% aqueous phosphoric acid solution to form anoxide film as a dielectric film on the tantalum sintered compact,thereby preparing a sample.

The sample was immersed in a 0.75 mol/l aqueous solution of pyrrole for2 minutes and then immersed in a 0.1 mol/l aqueous ferric sulfatesolution for 10 minutes. This operation was repeated about 20 times tocover the entire surface of the sample with polypyrrole by chemicalpolymerization. Then, the sample was washed with water, dried, immersedin a methanol solution of an ionic liquid, and then dried to removemethanol. By the above-described method, each ionic liquid was added tothe chemically polymerized polypyrrole layer to obtain an electrolyte ofthe present invention. The amount of the ionic liquid added was 0.5 to5% by weight of the conductive polymer. Next, a carbon layer and asilver paste layer were provided on the electrolyte of the presentinvention to prepare a capacitor. Then, a cathode lead was provided onthe silver paste layer, and the resultant capacitor of the presentinvention was aged at an applied voltage of 12.5 V for 1 hour. Next, thecapacitor was armored with a resin to prepare an electrolytic capacitor.The thus-prepared capacitor of the present invention was aged at 20 Vfor 1 hour. Then, the initial capacitance, tan δ, leakage current, andwithstand voltage (V) were measured.

Table 6 shows the characteristics of the resultant capacitors. Theinitial capacitance and tan δ were not so different from those of thecomparative example in which the ionic liquid was not added, but theleakage currents and the withstand voltages were significantly improved.It was thus found that improvement in the withstand voltage of anelectrolytic capacitor can be realized by applying the electrolyte ofthe present invention to the electrolytic capacitor, the electrolytebeing prepared by adding an ionic liquid to a conductive polymer. TABLE6 Initial properties of capacitor: tantalum/oxide film/(polypyrrole +ionic liquid) system Ionic Leakage Withstand liquid Capacitance tanδcurrent voltage Experiment No. added (μF) (%) (μA) (V) Example 49 ILS-116.7 1.6 0.13 18 Example 50 ILS-2 16.2 1.7 0.16 16 Example 51 ILS-3 17.01.8 0.12 18 Example 52 ILS-4 17.7 2.0 0.08 16 Example 53 ILS-5 16.6 1.70.10 16 Example 54 ILS-6 16.5 2.0 0.12 16 Example 55 ILS-7 17.0 1.9 0.0830 Example 56 ILS-8 16.8 1.8 0.09 20 Example 57 ILS-9 17.0 1.9 0.12 26Example 58 ILS-10 16.6 2.0 0.08 30 Example 59 ILS-11 16.4 2.0 0.08 30Example 60 ILS-12 16.5 2.0 0.13 28 Example 61 ILS-13 16.9 1.8 0.12 16Example 62 ILS-14 16.8 1.8 0.09 16 Example 63 ILS-15 17.5 1.7 0.12 16Example 64 ILS-16 17.0 1.8 0.08 20 Example 65 ILS-17 16.5 2.0 0.13 20Comparative No 17.2 1.8 0.18 12 Example 7

In particular, with the fluorine anion-containing ionic liquids (ILS-7to 12), significant improvements in the withstand voltages wereobserved. It was thus found that the ionic liquid of the presentinvention is effective in forming an oxide film on a tantalum metalsurface and repairing the oxide film.

Examples 66 to 82

A conductive polymer aluminum electrolytic capacitor (aluminumelectrolytic capacitor) was prepared using a conductive polymer formedon an aluminum oxide film by chemical polymerization of thiophene.

In other words, an aluminum etched foil of 4×3.3 mm was immersed in a 3%aqueous solution of ammonium adipate. Next, the voltage was increasedfrom 0 V to 10 V at a rate of 10 mV/sec, and a constant voltage of 10 Vwas applied for 40 minutes to form a dielectric film on the aluminumetched foil. Next, the foil was washed with flowing deionized water for10 minutes and then dried at 105° C. for 5 minutes. The capacitance ofthe resultant aluminum etched foil in the solution was 18 μF.

Next, an ethanol solution containing ferric benzenesulfonate which was atransition metal salt including benzenesulfonic acid anion and ferrictriisopropylnaphthalenesulfonate which was a transition metal saltincluding triisopropylnaphthalenesulfonic acid anion was prepared as anoxidizing agent. Next, 1,4-dioxythiophene was mixed with the oxidizingagent, and the resultant mixture was stirred to prepare a polymerizationsolution. The aluminum foil anodized as described above was immersed inthe solution, heated in an electric furnace at 105° C. for 5 seconds,further heated in an electric furnace at 70° C. for 10 minutes toprogress chemical polymerization, washed with deionized water, and thendried. This operation was repeated ten or more times so that the foilwas entirely covered with polythiophene as viewed with the eyes. Afterwashing and drying, each of the ionic liquids was added by the samemethod as in Example 28. (Namely, as described above, a polythiophenelayer was formed on a conductive layer by electrolytic polymerization,washed with water, dried, immersed in a methanol solution of each ionicliquid, and then dried to remove methanol. By this method, the ionicliquid was added to the electrolytically polymerized polypyrrole layerto prepare an electrolyte of the present invention. The amount of theionic liquid added was 0.5 to 5% by weight.) Then, a carbon layer and asilver paste layer were provided on the electrolyte of the presentinvention to prepare a capacitor. The characteristics of the resultantcapacitors are shown in Table 7. TABLE 7 Initial properties ofcapacitor: aluminum/oxide film/(polythiophene + ionic liquid) systemLeakage Withstand Experiment Ionic liquid Capacitance tanδ currentvoltage No. added (μF) (%) (μA) (V) Example 66 ILS-1 15.0 2.1 0.03 6Example 67 ILS-2 16.1 1.8 0.06 8 Example 68 ILS-3 15.0 2.4 0.10 8Example 69 ILS-4 15.0 2.0 0.04 8 Example 70 ILS-5 15.2 1.9 0.08 8Example 71 ILS-6 15.0 1.7 0.08 10 Example 72 ILS-7 15.0 1.8 0.04 10Example 73 ILS-8 15.0 2.2 0.09 8 Example 74 ILS-9 14.9 2.0 0.04 10Example 75 ILS-10 15.2 1.7 0.06 10 Example 76 ILS-11 15.2 1.5 0.05 10Example 77 ILS-12 15.0 1.8 0.06 10 Example 78 ILS-13 15.9 2.1 0.14 8Example 79 ILS-14 15.0 2.2 0.09 6 Example 80 ILS-15 15.5 2.2 0.11 8Example 81 ILS-16 15.0 2.8 0.08 8 Example 82 ILS-17 14.9 2.6 0.10 9Comparative No 15.2 2.0 0.28 4 Example 8

The initial capacitance and tan δ were not so different from those ofthe comparative example in which the ionic liquid was not added, but theleakage currents and the withstand voltages were significantly improved.It was thus found that improvement in the characteristics of anelectrolytic capacitor can be realized by applying the electrolyte ofthe present invention to the electrolytic capacitor, the electrolytebeing prepared by adding an ionic liquid to a conductive polymer. Inparticular, with the fluorine anion-containing ionic liquids (ILS-7 to12), significant improvements in the withstand voltages were observed,and a withstand voltage of 10 V corresponding to the initial formationvoltage was obtained.

Examples 83 to 87

The characteristics of a capacitor were evaluated using a systemcontaining the ionic liquid (ILS-2) and each of the solutes below at aweight ratio of 2:1. As the solutes added, the commercially availablesolutes below were used.

First, a methanol solution containing each of the solutes below andILS-2 at the above-described weight ratio was prepared, and a capacitorwas produced by the same method as in Examples 28 to 44. Namely, insteadof the step of immersing in the ILS-2 methanol solution in Example 29,the step of immersing in an ILS-2 methanol solution containing eachsolute was performed.

The characteristics of the resultant capacitors are shown in Table 8.TABLE 8 Initial properties of capacitor: aluminum/oxidefilm/(polypyrrole + ionic liquid + solute) system Ionic liquid WithstandExperiment added + Capacitance tanδ Impedance voltage No. solute added(μF) (%) (mΩ) (V) Example 83 ILS-2 + SA 4.5 1.2 94 32 Example 84 ILS-2 +SB 4.4 1.2 98 34 Example 85 ILS-2 + SC 4.5 1.2 94 30 Example 86 ILS-2 +SD 4.3 1.2 98 30 Example 87 ILS-2 + SE 4.6 1.3 94 32

Hereinafter, SA to SH each represent the solute. In Examples 83 to 87,solutes SA to SE were used respectively. However, solutes SF to SH wereused in subsequent examples and comparative examples, respectively.

Ammonium adipate (Diammonium adipate=(NH₄)⁺(⁻OOC—(CH₂)₄—COO⁻) (NH₄)⁺,abbreviated as “SA”)

Triethylamine maleate (Triethylammonium hydrogenmaleate=((C₂H₅)₃N—H)⁺(HOOC—CH═CH—COO)⁻, abbreviated as “SB”)

Tetraethylammonium maleate (Triethylammonium hydrogenmaleate=((C₂H₅)₄N)⁺(HOOC—CH═CH—COO)⁻, abbreviated as “SC”)

Tetraethylammonium phthalate (((C₂H₅)₄N)⁺(HOOC—C₆H₄—COO)⁻, abbreviatedas “SD”)

Tetraethylammonium benzoate (((C₂H₅)₄N)⁺(C₆H₅—COO)⁻, abbreviated as“SE”)

Triethylmethylammonium maleate (Triethylmethylammonium hydrogenmaleate=((C₂H₅)₃N—CH₃)⁺(HOOC—CH═CH—COO)⁻, abbreviated as “SF”)

Triethylmethylammonium phthalate (Triethylmethylammonium hydrogenphthalate=((C₂H₅)₃N—CH₃)⁺(1-HOOC—C₆H₄-2-COO)⁻, abbreviated as “SG”)

Phosphoric acid (H₃PO₄, abbreviated as “SH”)

Table 8 indicates that when an electrolyte of the present inventionprepared by adding an ionic liquid containing such a solute to aconductive polymer is applied to an electrolytic capacitor, thewithstand voltage of the capacitor can be further improved.

Examples 88 to 91

Table 9 shows the characteristics of capacitors prepared by the samemethod as in Examples 28 to 44 except the conditions (1) and (2) below.TABLE 9 Initial properties of capacitor: aluminum/oxidefilm/(polypyrrole + ionic liquid + solute) system Ionic liquid WithstandExperiment added + Capacitance tanδ Impedance voltage No. solute added(μF) (%) (mΩ) (V) Example 88 ILS-18 + SA 4.8 1.1 92 27 Example 89ILS-19 + SA 4.7 1.2 88 28 Example 90 ILS-20 + SA 4.6 1.2 92 23 Example91 ILS-21 + SA 4.7 1.3 96 22 Example 92 ILS-18 + SB 4.6 1.3 96 24Example 93 ILS-18 + SF 4.7 1.2 93 26 Example 94 ILS-18 + SG 4.8 1.2 9525 Comparative No 4.8 1.1 90 16 Example 4 Comparative ILS-22 + SA 4.71.8 390 4 Example 9 Comparative ILS-23 + SA 4.6 2.0 595 3 Example 10

(1) The ionic liquids ILS-18 to 21 were used, and any one of the solutesbelow was added to each of the ionic liquids (ILS-18 to 21) at a weightratio of 85:15 (the content of solute SA in the ionic liquid was 15%).In other words, in the process for producing a capacitor in each ofExamples 28 to 44, the step of immersing in each of ILS-18 to 21methanol solutions each containing the solute was performed instead ofthe step of immersing in the ILS-1 methanol solution in Example 28.

(2) In preparing a film by electrolytic polymerization, an electrolyteincluding methoxyphenol (0.15 M), pyrrole (0.5 M), an alcohol solutionof sodium triisopropylnaphthalenesulfonate (0.1 M), and water was usedinstead of the electrolyte including pyrrole (0.5 M), a 30% alcoholsolution of sodium triisopropylnaphthalenesulfonate (0.1 M), and water.

The characteristics of the resultant capacitors are shown in Table 9.The initial capacitance, tan δ, impedance value were not so differentfrom those of Comparative Example 4 in which the ionic liquid was notadded, but the withstand voltages were significantly improved. It wasthus found that improvement in the withstand voltage of a capacitor canbe realized by adding the ionic liquid.

Examples 92 to 94

An aluminum electrolytic capacitor was prepared by electrolyticpolymerization according to the same procedures as in Example 88. Asolution of the ionic liquid ILS-18 and 15% of each of the solutes SB,SF, and SG was added to the resultant electrolytic capacitor, and thecapacitor characteristics were measured. The results of the capacitorcharacteristics are shown in a lower part of Table 9. The initialcapacitance, tan δ, impedance value were not so different from those ofComparative Example 4 in which the ionic liquid was not added, but thewithstand voltages were significantly improved. It was thus found thatimprovement in the withstand voltage of a capacitor can be realizedregardless of the type of the solute added to the ionic liquid.

Comparative Examples 9 and 10

The characteristics of a capacitor were measured using each of the ionicliquids ILS-22 and 23 (in which 15% of SA was dissolved) by the samemethod as in Examples 88 to 91. The results are shown in Table 9. Theresults indicate that both the tan δ and the withstand voltage aresignificantly degraded. This is possibly due to bromine or chlorinepresent in the ionic liquid.

Examples 95 to 98

A tantalum conductive polymer electrolytic capacitor was prepared bychemical polymerization, and each of the ionic liquids ILS-18 to 21 (inwhich 15% of solute SA was dissolved) was added to the electrolyticcapacitor. The capacitor characteristics were measured.

Namely, a rectangular parallelepiped tantalum sintered compact (2 mm inlength, 1.5 mm in height, and 1 mm in width) provided with an anode leadwas subjected to anodization in a 0.05% aqueous phosphoric acid solutionat 85° C. for 60 minutes with a voltage of 33.9 V applied to form adielectric film. The resultant element was immersed in a 0.75 mol/laqueous solution of pyrrole for 2 minutes and then immersed in a 0.1mol/l aqueous solution of ferric sulfate for 10 minutes. This operationwas repeated about 20 times to cover the entirety of the element with aconductive polypyrrole polymer by chemical oxidation polymerization.Next, the ionic liquid was added by the same method as in Example 45.

Next, a carbon paste film and a silver paste film were formed by anordinary method, and a cathode is provided on the silver paste film. Theelement was aged with a voltage of 12.5 V applied and armored with aresin to prepare an electrolytic capacitor. The resultant capacitor ofthe present invention was aged at 20 V for 1 hour, and then the initialcapacitance, tan δ, leakage current, and the withstand voltage (V) weremeasured.

Table 10 shows the characteristics of the resultant capacitors. Theinitial capacitance and tan δ were not so different from those ofComparative Example 11 in which the ionic liquid was not added, but theleakage currents and the withstand voltages were significantly improved.It was thus found that improvement in the characteristics of a capacitorcan be realized by adding the ionic liquid. TABLE 10 Initial propertiesof capacitor: tantalum/oxide film/(polypyrrole + ionic liquid + solute)system Leakage Experiment Ionic liquid Capacitance tanδ currentWithstand No. added (μF) (%) (μA) voltage (V) Example 95 ILS-18 + SA16.6 2.2 0.09 19 Example 96 ILS-19 + SA 16.2 2.2 0.19 18 Example 97ILS-20 + SA 16.4 2.3 0.26 16 Example 98 ILS-21 + SA 16.0 2.4 0.18 16Comparative No 17.2 2.0 0.18 12 Example 11

Examples 99 to 102

A conductive polymer aluminum electrolytic capacitor was prepared bychemical polymerization of thiophene, and each of the ionic liquidsILS-18 to 20 (in which 15% of solute SA was dissolved) was added to theresultant electrolytic capacitor. The characteristics of the resultantcapacitors were measured.

Namely, an aluminum etched foil of 4×3.3 mm was immersed in a 3% aqueoussolution of ammonium adipate. Next, the voltage was increased from 0 Vto 10 V at a rate of 10 mV/sec, and a constant voltage of 10 V wasapplied for 40 minutes to form a dielectric film on the aluminum etchedfoil. Next, the foil was washed with flowing deionized water for 10minutes and then dried at 105° C. for 5 minutes. The capacitance of theresultant aluminum etched foil in the solution was 18 μF.

Next, an ethanol solution containing ferric benzenesulfonate which was atransition metal salt including benzenesulfonic acid anion and ferrictriisopropylnaphthalenesulfonate which was a transition metal saltincluding triisopropylnaphthalenesulfonic acid anion was prepared as anoxidizing agent. Next, 1,4-dioxythiophene was mixed with the oxidizingagent, and the resultant mixture was stirred to prepare a polymerizationsolution. The aluminum foil anodized as described above was immersed inthe solution, heated in an electric furnace at 105° C. for 5 seconds,further heated in an electric furnace at 70° C. for 10 minutes to allowchemical polymerization to proceed, washed with deionized water, andthen dried. This operation was repeated so that the aluminum foil wasentirely covered with polythiophene. After washing and drying, eachionic liquid was added by the same method as in Example 1. Then, acathode was formed using carbon paste and silver paste to prepare acapacitor.

The characteristics of the resultant capacitors are shown in Table 11.The initial capacitance and tan δ were not so different from those ofComparative Example 12 in which the ionic liquid was not added, but theleakage currents and the withstand voltages were significantly improved.It was thus found that improvement in the characteristics of a capacitorcan be realized by adding the ionic liquid. TABLE 11 Initial propertiesof capacitor: aluminum/oxide film/(polythiophene + ionic liquid +solute) system Leakage Withstand Experiment Ionic liquid Capacitancetanδ current voltage No. added (μF) (%) (μA) (V) Example 99 ILS-18 + SA15.0 2.2 0.09 8 Example 100 ILS-19 + SA 14.8 2.2 0.15 9 Example 101ILS-20 + SA 14.6 2.3 0.16 7 Example 102 ILS-21 + SA 14.5 1.7 0.06 6Comparative No 15.2 2.0 0.28 4 Example 12

Examples 103 to 119

An electrolytic capacitor was experimentally prepared using a TCNQ saltformed on an aluminum oxide film by melting impregnation, and each ionicliquid was added to the resultant electrolytic capacitor. Thecharacteristics of the capacitors were measured.

Namely, an aluminum foil etched at a high surface magnification wasimmersed in a 3% aqueous solution of ammonium adipate and subjected toanodization at 70° C. with a voltage of 50 V applied to form an oxidefilm as a dielectric film on the surface of the aluminum foil. Thealuminum foil was used as an anode foil/cathode foil, and a lead wirewas attached thereto. Then, the foil was coiled through Manila hemppaper used as a separator to form a coiled capacitor element. Next, inorder to facilitate impregnation of the molten TCNQ salt, the capacitorelement was heated to carbonize the separator paper.

Next, an armoring aluminum case was filled with anN-n-butylisoquinolinium (TCNQ)₂ salt and each ionic liquid (2%),followed by melting at 210° C. Then, the pre-heated capacitor elementwas placed in the case, and the aluminum case was cooled with liquidnitrogen immediately after the element was placed in the case.

Next, an epoxy resin was injected into an upper portion of the case andthen heat-cured to seal the case. The thus-prepared capacitor of thepresent invention was aged at 20 V for 1 hour, and then the initialvoltage, tan δ, impedance (120 Hz), and the withstand voltage (V) weremeasured. The withstand voltage was measured as a voltage at which theleakage current started to increase with increases in the voltage at aconstant rate. Since measurement of the withstand voltage produced alarge error, the measured values of ten or more elements were averagedto determine the withstand voltage. The characteristics of the resultantcapacitors are shown in Table 12. The ionic liquids used were ILS-1 to17. TABLE 12 Initial properties of capacitor: aluminum/oxidefilm/(N-n-butylisoquinolinium (TCNQ)2 salt + ionic liquid) systemWithstand Experiment Ionic liquid Capacitance tanδ Impedance voltage No.added (μF) (%) (mΩ) (V) Example 103 ILS-1 6.6 2.2 98 28 Example 104ILS-2 6.5 2.2 97 27 Example 105 ILS-3 6.8 2.2 92 26 Example 106 ILS-46.6 2.2 92 25 Example 107 ILS-5 6.8 2.1 90 25 Example 108 ILS-6 6.7 2.293 26 Example 109 ILS-7 6.7 2.2 98 38 Example 110 ILS-8 6.8 2.1 90 25Example 111 ILS-9 6.7 2.3 89 38 Example 112 ILS-10 6.9 2.1 92 44 Example113 ILS-11 6.8 2.2 105 42 Example 114 ILS-12 6.7 2.3 102 38 Example 115ILS-13 6.7 2.2 98 28 Example 116 ILS-14 6.8 2.1 90 25 Example 117 ILS-156.7 2.1 89 24 Example 118 ILS-16 6.8 2.2 95 24 Example 119 ILS-17 6.62.1 109 28 Comparative No 6.9 2.1 82 18 Example 13

The initial capacitance, tan δ, and impedance value were not sodifferent from those of Comparative Example 13 in which the ionic liquidwas not added, but the withstand voltages were significantly improved.It was thus found that improvement in the withstand voltage of anelectrolytic capacitor can be realized by applying the electrolyte ofthe present invention to the electrolytic capacitor, the electrolytebeing prepared by adding an ionic liquid to a TCNQ salt.

Examples 120 to 123

An aluminum electrolytic capacitor was experimentally prepared using aTCNQ salt electrolyte formed by melting impregnation according to thesame procedures as in Examples 103 to 119. Then, each of the ionicliquids ILS-18 to 21 was added to the resultant electrolytic capacitor,and the characteristics of the capacitors were measured.

The characteristics of the resultant capacitors are shown in table 13.TABLE 13 Initial properties of capacitor: aluminum/oxidefilm/(N-n-butylisoquinolinium (TCNQ)2 salt + ionic liquid) system IonicWithstand Experiment liquid Capacitance tanδ Impedance voltage No. added(μF) (%) (mΩ) (V) Example 120 ILS-18 6.8 2.2 90 25 Example 121 ILS-196.7 2.2 98 28 Example 122 ILS-20 6.7 2.3 92 19 Example 123 ILS-21 6.42.5 97 17 Comparative No 6.8 2.1 92 16 Example 14 Comparative ILS-22 7.63.4 870 0.1 Example 15 Comparative ILS-23 6.3 4.4 1700 0.2 Example 16

With the hydrophilic ionic liquids ILS-18 and 19, the initialcapacitance, tan δ, and impedance value were not so different from thoseof Comparative Example 14 in which the ionic liquid was not added (TCNQsalt was used as an electrolyte), but the withstand voltage wassignificantly improved. It was thus found that improvement in thewithstand voltage of a capacitor can be realized by adding the ionicliquid.

On the other hand, with the hydrophobic liquids ILS-20 and 21,substantially no influence on the capacitor characteristics wasobserved, and the withstand voltage was slightly improved. This ispossibly due to the low anodizing ability of the ILS-21 and 20.

Comparative Examples 15 and 16

Each of ILS-22 and 23 was added by the same method as in Example 120,and the capacitor characteristics were measured. The results ofmeasurement are shown in a lower part of Table 13. It was found thatwhen a chlorine- or bromine-containing ionic liquid is added, thecapacitor characteristics are significantly degraded.

Examples 124 to 131

An electrolytic capacitor was produced by the same method as in Example103 except that N-isoamylisoquinolinium (TCNQ)₂ salt was used in placeof N-n-butylisoquinolinium (TCNQ)₂ salt. The melting impregnationtemperature was 215° C. The characteristics of the resultant capacitorsare shown in Table 14. In the experiment, the ionic liquids ILS-1 to 5,10, 13, and 14 were used. TABLE 14 Initial properties of capacitor:aluminum/oxide film/(N-n-isoamylisoquinolinium (TCNQ)2 salt + ionicliquid) system Withstand Experiment Ionic liquid Capacitance tanδImpedance voltage No. added (μF) (%) (mΩ) (V) Example 124 ILS-1 6.8 2.6102 26 Example 125 ILS-2 7.6 2.8 104 26 Example 126 ILS-3 7.2 3.4 98 28Example 127 ILS-4 7.2 3.4 98 28 Example 128 ILS-5 6.8 2.6 102 26 Example129 ILS-10 7.2 2.9 104 36 Example 130 ILS-13 7.6 2.8 104 26 Example 131ILS-14 6.6 2.5 94 25 Comparative 7.8 2.4 90 18 Example 17

The initial capacitance, tan δ, and impedance value were not sodifferent from those of the comparative example in which the ionicliquid was not added, but the withstand voltages were significantlyimproved. It was thus found that improvement in the withstand voltage ofan electrolytic capacitor can be realized by applying the electrolyte ofthe present invention, which contains the ionic liquid, to theelectrolytic capacitor regardless of the type of the TCNQ salt used.

Examples 132 to 135

A TCNQ salt electrolyte was formed by adding 2% of each of the ionicliquids ILS-18 to 21 by melting impregnation, and an aluminumelectrolytic capacitor was experimentally produced by the same method asin Example 103 except that N-isoamylisoquinolinium (TCNQ)₂ salt was usedin place of N-n-butylisoquinolinium (TCNQ)₂ salt as in Examples 124 to131. The characteristics of the capacitors were measured. In thisexperiment, the N-isoamylisoquinolinium (TCNQ)₂ salt (salt B) was moltenand impregnated at 215° C. The characteristics of the resultantcapacitors are shown in Table 15. TABLE 15 Initial properties ofcapacitor: aluminum/oxidefilm/(N-n-isoamylisoquinoliniumbutylisoquinolinium (TCNQ)2 salt + ionicliquid) system Withstand Experiment Ionic liquid Capacitance tanδImpedance voltage No. added (μF) (%) (mΩ) (V) Example 132 ILS-18 6.8 2.590 27 Example 133 ILS-19 6.4 2.2 95 29 Example 134 ILS-20 6.6 2.3 98 20Example 135 ILS-21 6.2 2.3 106 18

With the hydrophilic ionic liquids ILS-18 and 19, the initialcapacitance, tan δ, and impedance value were not so different from thoseof Comparative Example 17 in which the ionic liquid was not added, butthe withstand voltages were significantly improved. It was thus foundthat improvement in the withstand voltage of a capacitor can be realizedby adding the ionic liquid. On the other hand, with the hydrophobicliquids ILS-20 and 21, substantially no influence on the capacitorcharacteristics was observed, and the withstand voltages were slightlyimproved. This is possibly due to the low anodizing ability of theILS-21 and 20 as compared with the ILS-18 and 19.

Examples 136 to 140

Each of the solutes (SA to SE) was added to the ionic liquid (ILS-2) sothat the weight ratio between the ionic liquid and the solute was 4:1(the content of solute SA dissolved in the ionic liquid was 20%). Acapacitor was produced by the same method as in Example 103. Thecharacteristics of the resultant capacitors are shown in Table 16. TABLE16 Initial properties of capacitor: aluminum/oxide film/(N-n-butylisoquinolinium (TCNQ)2 salt + ionic liquid + solute) systemIonic liquid Withstand Experiment added + Capacitance tanδ Impedancevoltage No. solute added (μF) (%) (mΩ) (V) Example 136 ILS-2 + SA 6.53.2 96 33 Example 137 ILS-2 + SB 6.0 3.8 97 30 Example 138 ILS-2 + SC5.5 3.7 104 28 Example 139 ILS-2 + SD 5.8 3.5 96 30 Example 140 ILS-2 +SE 5.7 3.0 108 32

The table indicates that when the electrolyte of the present invention,which is prepared by adding the ionic liquid containing such a solute toTCNQ salt, is applied to an electrolyte capacitor, the withstand voltagethereof can be further improved.

Examples 141 to 144

A TCNQ salt electrolyte was formed by adding 5% of each of the ionicliquids ILS-18 to 21 (in which 15% of solute SA was dissolved) bymelting impregnation, and an aluminum electrolytic capacitor wasexperimentally produced by the same method as in Example 120. Thecharacteristics of the capacitors were measured.

The characteristics of the resultant capacitors are shown in Table 17.The initial capacitance, tan δ, and impedance value were not sodifferent from those of Comparative Example 17 in which the ionic liquidwas not added, but the withstand voltages were significantly improved.It was thus found that improvement in the withstand voltage of acapacitor can be realized by adding the ionic liquid containing thesolute dissolved therein. TABLE 17 Initial properties of capacitor:aluminum/oxide film/ (N-n-butylisoquinolinium (TCNQ)2 salt + ionicliquid + solute) system Ionic liquid Withstand Experiment added +Capacitance tanδ Impedance voltage No. solute added (μF) (%) (mΩ) (V)Example 141 ILS-18 + SA 6.6 2.2 97 28 Example 142 ILS-19 + SA 6.4 2.5 9032 Example 143 ILS-20 + SA 6.5 2.3 108 28 Example 144 ILS-21 + SA 6.42.5 95 27

Examples 145 to 147

An aluminum electrolytic capacitor was experimentally produced by thesame method as in Example 120, and a solution prepared by dissolving 15%of each of the solutes SB, SG, and SF to the ionic liquid ILS-18 wasadded to the electrolytic capacitor. The characteristics of thecapacitors were measured. The characteristics of the resultantcapacitors are shown in Table 18. The initial capacitance, tan δ, andimpedance value were not so different from those of Comparative Example17 in which the ionic liquid was not added, but the withstand voltagewas significantly improved. It was thus found that improvement in thewithstand voltage of a capacitor can be realized regardless of the typeof the solute added to the ionic liquid. TABLE 18 Initial properties ofcapacitor: aluminum/oxide film/ (N-n-butylisoquinolinium (TCNQ)2 salt +ionic liquid + solute) system Ionic liquid Withstand Experiment added +Capacitance tanδ Impedance voltage No. solute added (μF) (%) (mΩ) (V)Example 145 ILS-18 + SF 6.4 2.7 94 29 Example 146 ILS-18 + SG 6.0 2.5109 23 Example 147 ILS-18 + SB 6.4 2.6 94 26

Examples 148 to 159

Experiments were conducted by changing the amount of the ionic liquidadded by the same method as in Examples 28 and 37. The results are shownin Table 19. TABLE 19 Initial properties of capacitor: aluminum/oxidefilm/(polypyrrole + ionic liquid) system, influence of amount of ionicliquid added Ionic liquid added Initial properties Adding Capac-Withstand Ionic amount itance tanδ Impedance voltage liquid (%) (μF) (%)(mΩ) (V) Example 148 ILS-1 0.01 4.7 1.1 92 20 Example 149 0.1 4.8 1.2 9524 Example 150 2 4.6 1.2 95 28 Example 151 10 4.5 1.5 162 30 Example 15220 4.3 1.9 390 38 Example 153 50 4.0 2.8 >500 38 Example 154 ILS-10 0.014.7 1.1 95 22 Example 155 0.1 4.6 1.2 96 28 Example 156 2 4.6 1.3 99 34Example 157 10 4.5 1.8 180 38 Example 158 20 4.4 2.4 470 42 Example 15950 4.0 3.4 >500 45 Comparative No 4.8 1.1 90 16 Example 4

The results indicate that even when the amount of the ionic liquid addedto a conductive polymer (polypyrrole) is 0.01 part by weight relative to100 parts by weight of the conductive polymer, there is the effect ofimproving the withstand voltage, and the effect is significant with theionic liquid added in an amount of 0.1 part by weight or more. However,when the amount is 10 parts by weight or more, the impedance property isdegraded, and the capacitance also tends to be decreased.

The same tendency applies to a case in which tantalum is used as anelectrode, a case in which a conductive polymer such as a thiophenepolymer other than a pyrrole polymer is used as the conductive polymer,and a case in which either of electrolytic polymerization and chemicalpolymerization is used. These results reveal that in an electrolyteincluding a conductive polymer and an ionic liquid, the amount of theionic liquid added is most preferably in a range of 0.01% to less than10% relative to 100% by weight of the conductive polymer.

Examples 160 to 169

Experiments were conducted using a TCNQ salt by the same method as inExamples 103 and 112 except that the amount of the ionic liquid addedwas changed to various values. The results are shown in Table 20. TABLE20 Initial properties of capacitor: aluminum/oxidefilm/(N-n-butylisoquinolinium (TCNQ)2 salt + ionic liquid) system,influence of amount of ionic liquid added Ionic liquid added Initialproperties Adding Capac- Withstand Ionic amount itance tanδ Impedancevoltage liquid (%) (μF) (%) (mΩ) (V) Example 160 ILS-1 0.01 6.7 2.1 9720 Example 161 0.1 6.8 2.2 97 24 Example 103 2 6.6 2.2 98 28 Example 16210 6.4 2.5 146 30 Example 105 20 5.3 3.9 240 36 Example 106 50 4.85.8 >500 38 Example 107 ILS-10 0.01 6.7 2.1 95 24 Example 108 0.1 6.62.2 94 32 Example 78 2 6.9 2.1 92 44 Example 110 10 6.2 2.8 230 46Example 111 20 5.7 3.4 470 48 Example 112 50 4.2 5.4 >500 >50Comparative No 6.9 2.1 82 18 Example 13

The results indicate that even when the amount of the ionic liquid addedto a TCNQ salt is 0.01 part by weight relative to 100 parts by weight ofthe TCNQ salt, there is the effect of improving the withstand voltage,and the effect is significant with the ionic liquid added in an amountof 0.1 part by weight or more. However, when the amount is 10 parts byweight or more, the impedance property is degraded, and the capacitancealso tends to be decreased.

These results reveal that in an electrolyte including a TCNQ salt and anionic liquid, the amount of the ionic liquid added is most preferably ina range of 0.01% to less than 10% relative to 100% by weight of the TCNQsalt.

INDUSTRIAL APPLICABILITY

By using the method of the present invention, an oxide film can beeasily formed on a valve metal. Furthermore, when the method of thepresent invention is used for an electrolyte of an electrolyticcapacitor, a high-performance electrolytic capacitor having excellenthigh-frequency characteristics and a high withstand voltage can beobtained on the basis of the nonvolatility and excellent oxide filmrepairing ability of an ionic liquid contained in the electrolyte.

1. A method for forming an oxide film on a metal surface, the methodcomprising anodization in the presence of an ionic liquid.
 2. The methodfor forming an oxide film on a metal surface according to claim 1,wherein a defect of an oxide film previously formed on a metal surfaceis repaired by the anodization in the presence of an ionic liquid. 3.The method for forming an oxide film on a metal surface by anodizationaccording to claim 1, wherein the metal is at least one selected fromaluminum and/or alloys thereof, tantalum and/or alloys thereof, andniobium and/or alloys thereof.
 4. The method for forming an oxide filmon a metal surface according to claim 1, wherein an anion component ofthe ionic liquid is an atomic group containing fluorine.
 5. The methodfor forming an oxide film on a metal surface according to claim 1,wherein an anion compound of the ionic liquid is an atomic groupcontaining a sulfonic acid anion (—SO₃ ⁻).
 6. The method for forming anoxide film on a metal surface by anodization according to claim 1,wherein an anion component of the ionic liquid is an atomic groupcontaining a carboxylate anion (—COO⁻).
 7. The method for forming anoxide film on a metal surface according to claim 1, wherein a cationcomponent of the ionic liquid is at least one selected from imidazoliumderivatives, ammonium derivatives, and pyridinium derivatives.
 8. Themethod for forming an oxide film on a metal surface by anodizationaccording to claim 1, wherein a solution containing an ionic liquid andat least one selected from ammonium salts, amine salts, quaternaryammonium salts, tertiary amines, and organic acids is used.
 9. Anelectrolytic capacitor comprising means for the method according toclaim 1 for repairing an oxide film.
 10. An electrolytic capacitorcomprising a solution containing at least one ionic liquid and used asan electrolyte serving as means for repairing an oxide film.
 11. Theelectrolytic capacitor according to claim 10, wherein the solutionfurther contains a conductive polymer.
 12. The electrolytic capacitoraccording to claim 11, wherein the conductive polymer is at least oneselected from polypyrrole, polyaniline, polythiophene, and derivativesthereof.
 13. The electrolytic capacitor according to claim 11, whereinthe weight ratio (ionic liquid/conductive polymer) of the ionic liquidto the conductive polymer is in a range of 1/10,000 to less than 1/10.14. The electrolytic capacitor according claim 10, wherein the solutionfurther contains a TCNQ salt.
 15. The electrolytic capacitor accordingto claim 14, wherein the TCNQ salt is a salt containing a donor composedof a nitrogen-containing heterocyclic compound substituted by an alkylat the N position and an acceptor composed of TCNQ.
 16. The electrolyticcapacitor according to claim 10, wherein an anion component of the ionicliquid is an atomic group containing at least fluorine.
 17. Theelectrolytic capacitor according to claim 10, wherein an anion componentof the ionic liquid is an atomic group containing at least a sulfonicacid anion (—SO₃ ⁻).
 18. The electrolytic capacitor according to claim10, wherein an anion component of the ionic liquid is an atomic groupcontaining at least a carboxylate anion (—COO⁻).
 19. The electrolyticcapacitor according to claim 14, wherein the weight ratio (ionicliquid/TCNQ salt) of the ionic liquid to the TCNQ salt is in a range of1/10,000 to less than 1/2.
 20. The electrolytic capacitor according toclaim 10, wherein a cation component of the ionic liquid is animidazolium derivative, an ammonium derivative, or a pyridiniumderivative.
 21. An electrolyte comprising a solution containing theionic liquid according to claim 1, wherein the electrolyte is used forforming an oxide film on a metal surface by anodization.
 22. Anelectrolyte comprising a solution containing the ionic liquid accordingto claim 9, wherein the electrolyte is used for an electrolyticcapacitor.